I feed 20% protein and haven't had a single one with any leg problems. I keep mine in a 6x10' tractor and I've only done 25 at a time- it's plenty big for that many. Males will be huge at 8 weeks, so I like to take them out at 6-7 weeks. (I just don't need more than one or two birds that go 7.5+ lbs fully dressed.) The females take a little longer to get that big.
The average weight for the last batch I did was a little over 6 lbs dressed at 8 weeks. I'm not sure I fully agree with rushing to butcher them all if one dies- that's what the Cornish X's do. Although, I certainly understand the feeling of not wanting to lose any more, so you'll have to use your best judgment on that one. You can plan on a couple dying, though, and from the batches I've gotten, they die in the last four weeks much more often than the first four.