No you aren't way off base. :) Basically you hit the nail on the head. Suspended no more than 2" above the plants, the effective foot print of any fixture is slightly less than the dimensions of the fixture itself. So a 4'x2' has approx. a 3.75x1.75 effective footprint.

But "adequately" is the key word. Defining it when growing seedlings that are acceptable for garden transplants is very different from defining it when using artificial light to provide optimal growth, bloom, and fruit indoors. So I'll assume you are talking about growing seedling transplants for the garden. If not then the Growing Under Light forum is the place to go.

And there are other variables. T8 bulb fixture are only marginally better than the old T12 fixtures so for all practical purposes the effective footprint is the same. T5 fixtures are measurably better than T8 and allow them to hang a couple of inches higher and still be as effective.

How much does that added height of the T5 increase your effective foot print? Not much. It basically eliminates the dead space loss at the ends of the tubes and brings you up to full fixture dimensions. The fixture ends will still be weaker - more equivalent to T8 - but adequate.

There are other factors of course - types of bulbs, other ambient light, age of the bulbs, etc. but for all practical purposes when using fluorescent tube light fixtures, the "adequacy" footprint is the size of the fixture itself. To improve on that you have to move into the high power fixtures, the big CFLs, HPS, and metal halide. They will have much bigger footprints than the fixture size.

A simple rule of thumb when growing seedling transplants is "more is better and closer is better". You can easily have too little light but you can seldom have too much light.

Dave

Wow, I wasn't expected that the light footprint was going to be so close to the fixture size.

I was hoping that a four bulb fixture with the more powerful T5 or High output T5 would be able to site a bit higher and maybe hit a space 50% larger than the fixture and do the job.

I am sure Dave is right about the footprint, but practically speaking, two double lamp fixtures (4 bulbs) are what most people use successfully for 4 standard sized trays of seedlings. With the narrower t-8 fixtures, some of my shelves have 3 double bulb lamps, but I am not seeing a big difference in the plant health / size., Nor do I see much difference in the plants that are "between" the bulbs, as opposed to directly under them.

I second Mandolls' thoughts. Here is my set-up.
{{gwi:234339}}
No issues with 2 sets of t8's lighting 4 standard and 1 half tray. The trays the peppers are in are smaller than standard trays so I can easily light 5 of those trays. I too don't see much difference in the plants that fall between light bulbs.

Don't overthink it :)