It depends on the cleanliness of the operation. If you were to completely sterilize the straw, 1% would be more than enough. Commercial operations use close to 10%, but 5% is a good number. You next spawn is 1/20 of the current batch (if successful), so it becomes a bit like doing sourdough bread. Unlike sourdough bread, though, your starter will last a few months at least.

Let me mention that I have never been successful with in-drum pasteurization (0/2), and always been successful with pressure cooker sterilization (I have a 4 gallons one) (4/4) and 1% inoculation (basically, 2 plugs for 1 qt jar).

Glib thanks for the information. Just to make sure I am understanding these prcentages correctly, the percentages are ratio of the weight of the spawn to the weight of the growing medium. Is that correct?

Yes.

How exactly are you going to do this? Just inoculate the spores in the 55 gal, and wait for full colonization? If so, that will take quite a while. I use the widemouth 8 oz mason jars and inject a mL or so of spore solution, it tajes about a month to fully colonize. I can imagine how long 55 gal will take.

this is spawn, not spores. Typically, span is fully colonized sawdust.

Sorry, i misunderstood. Certainly, spawn will be quicker than spores, but 55 gal isn't small, it'll still take a few weeks.. Depending.. Research Paul Stamets, you will find all the information you need to know from his books,etc. Still, i would probably use smaller containers, not only for quicker colonization, but you are not leaving all your food on one plate. What happens if that 55 gal gets contaminated, it's garbage. What happens if a few of your 50 gal containers get contaminated, you still have the other 40! Plus, it's easier sterilizing small amounts of soil.