I know you would like an actual growth dimension per year, but there are too many variables involved. The U.S. National Arboretum states that the Acoma will grow to only "9.5 feet tall by 11 feet wide at 15 years". Your first year will probably have limited growth, followed by a few years of more rapid growth, and then the tree will probably kinda crawl into the last 9.5 feet during the final years of the 15 year window.
You can feed the tree Nitrogen in the spring to encourage growth, but it may be at the expense of early flowering. Nitrogen at this time of year would be a bad thing since the new growth may not harden off before your first freeze.

Here is a link that might be useful: Lagerstroemia 'Acoma'

Maybe you have already perused some of this information, maybe not. If not, you should. This plant arises from the unparalleled work of the late Dr. Don Egolf to hybridize, evaluate, select, and introduce improved Lagerstroemia for landscape use.

http://www.usna.usda.gov/Research/Herbarium/Lagerstroemia/Checklist_A.html

http://www.usna.usda.gov/graphics/usna/Newintro/USNA_CrapeMyrtlePoster.pdf

Based on all this information and my personal experience with 'Acoma' since the late 1980s in central Kentucky, you should achieve about one foot (12", 30 cm) of growth on average. It really does have this semipendulous habit as shown and described; I'd call it an umbrella shape.

Here is an image from Louisville. This 'Acoma' has been growing at this site and flowering its fool head off since about 2004. This image is from 2010. It was a 10G plant at installation - maybe 5 feet tall.

Here is a link that might be useful: USNA fact sheet for Lagerstroemia 'Acoma'

"I've tried searching for growth rate and all I can find is mature size and medium/fast grower. I'd like to know how many inches on average will the Acoma grow per year in normal conditions?"

I've tried searching for what times birds chirp and all I can find is information that gives general time periods, like morning or evening. I'd like to know which minutes of the day on average that birds will chirp in "normal conditions"?

I'm not trying to be a smart-alec, but just using the example to show how your question is not really answerable, especially because "normal conditions" don't actually exist.

My question may not seem answerable due to the way it was worded. However most everything can be broken down to an average.

Lets delve into your non-smarty-booty response. Granted knowing the exact minutes birds chirp is silly. What is the average time of day they chirp is still silly. My question wasn't that silly. For comparitive reasons we will go with active, over chirps. What time a day are birds most active. So if we were to ask an ornithologist "on average what time of day are birds most active?", he would probably say, too many variables with the different species. Well no problem, lets break it down to the Night Heron. Just like I broke it down to an Acoma Crepe. An Ornithologist would then say, on average Night Herons are most active in the late evenings.

Granted this average may not be what I experience based on my location, soil, quality of plant and all the other varibles that could negatively or positively affect that average. But an average growth to this tree does exist, based on average growing conditions, for the average gardener. That is what I wanted to know, the basic average of this trees growth. Don't need to get into all the minute details that may sway it one way or the other, off the average. Just the everything being equal, normal conditions, average that can be expected. That is my non-smarty-booty response. Really, a non- smarty-booty response!

crape myrtles generally grow pretty fast.

If you go to this website:
http://crapemyrtleguy.com

you can contact him through email or text and he will be able to give you a good answer.
And he responds very quickly.
The fastest growing crape myrtle is the natchez.
It is also white but grows much bigger.
Good luck!

"But an average growth to this tree does exist, based on average growing conditions, for the average gardener."

To my knowledge, this information is not known for ANY type of tree, and, would require a VAST amount of data to be analyzed. While it might be technically knowable with enough research, such research would be impractical and has never been attempted. The figures you see on nursery tags, for example, are not meant to truly represent an average. These type of figures are supposed to represent a typical (not average) growth rate, but are often way, way different than what most gardeners actually experience. Maybe "typical" is what you are actually after, but typical is probably best left to generalized terms like "medium/fast grower". I don't think you'll find anything more accurate than what Gregorywoodl's link gave, although those figures are unlikely to reflect your eventual experience.

"...he will be able to give you a good answer."

Crapemyrtleguy is a highschool teacher with a relatively newly-found hobby of growing crApe myrtles. I think the National Arboretum's (they introduced 'Acoma') figures would be as good as it's going to get for typical growth rates.

Thank you for all the links. Has provided some decent reading I had not seen before. Originally wanted the Nachez CrEpe, but it wasn't available. In it's spot, picked up two Sarahs Favorite Whites. Hopefully they prove to be a decent switch. Looked beautiful this year blooming and grew an easy 15" since planting. The Acoma, maybe 4". That is what led me to the question.

Brandon something, not sure what, gives me the feeling you like to debate. So here is a question, a genuine one. Not trying to be a smarty b this time. What is right crepe or crape? I've seen both listed on tags, in mags, and throughout the web. But what was originial?

Dr. Carl Whitcomb spells it "Crapemyrtle". I have to go with Dr. Whitcomb; he is the creator of the popular Dynamite, Red Rocket, Pink Velour, Rhapsody in Pink, Burgundy Cotton, and so on and so on.

Ironically, spell check tells me that both "Whitcomb" and "crapemyrtle" are spelled incorrectly.

Here is a link that might be useful: Crapemyrtle, Crape Myrtle, Crepe Myrtle �

More on crape myrtle vs. crepe myrtle.

Here is a link that might be useful: crape v. crepe

Cercis's link does a pretty good job covering the "whys". I could ad more support for the "a" spelling, but it would mostly be superfluous.

To me though, it's not as much about the why as it is about the results you get by using the preferred (by most almost all experts in the horticultural field) spelling. Just a few years ago, the a spelling gave far more Google search results. That has actually flip-flopped in just the last few years. Still, most of the instance of e-spelling is from amateurs (where the word just comes up in conversation). Most technical sources (scientific papers, publications from educational institutions, professional growers, etc) use the a-spelling. If everyone would suddenly agree on one single way, I'd be fine even if we spelled it with a "y". For now though, I find it helpful to stick with the "a".

Actually, the experts probably prefer lagerstroemia ;-)

You'd think, but that seems mostly not to be the case. I frequently wonder why, even in much academic/scientific work, common names are still far more common than the scientific names. Maybe they're afraid that some botanist will come along and change the scientific name next week.

I was 12 the first time I saw a CrApe(suppose i'll use the proper one) Mrytle in Charleston SC. Remember asking the family friend about it, she introduced it as a crEpe. She had a beautiful garden. Was probably also enhanced by the fact I was visiting her from AZ where we had a rock and cactus garden. Thank you for the info. I'll adjust my spelling of it from now on.