One thing that's been enlightening to me is doing a "trial run" with just a few birds. I ordered a few with my last layer order (or you can buy a few chicks locally sometimes) and now I have 4 birds at 7 weeks in my 8x6 tractor. It's given me some ideas for modifications to make feed management, etc. easier. Much better to get your method zeroed in with a low load than when you have it at full capacity. It's also been helpfull to get an idea of how many I think my space can hold, how often I'm gonna have to move it, etc.
So far it looks like I'm planning on starting with 25 birds, then butchering 5 or 6 at 2.5-3 lbs for use as small cornish hens (anyone here do this?). Then I may process a few more at 6-7 weeks, if necessary, then the rest at 8 weeks or so. They really seem to take off at 5-6 weeks, so taking some out at 6-7 weeks may help free up some space and keep me from having to move the tractor 3 times a day. It'll also help to keep me from having to process so many birds at one time.