I also use FF for my layers, but for meaties, here it goes:
--Cornish Cross, 25 to 30 birds
--For first 10 days: Fermented Purina chick starter (non-medicated), available all the time.
--After 10 days: Fermented Flock Raiser (which is 20% protein). I feed them twice a day, all they can eat in 15 minutes. Basically I give enough so that all the chickens have had their fill and start wandering away from the trough. The actual amount varies and gets quite a bit larger as they grow. I supplement that with a few cups of sprouted fodder and old garden produce which I throw into the yard away for their normal feeding area to encourage them to move.
--Free range within the confines of a very large fenced area. I live in an arid area, so there's not a lot of grass, but they do chase bugs, eat some weeds, and "forage" the produce I put in there.
I should add that my preference is to let them grow out to 11 to 13 weeks before I start butchering, which isn't quite the norm. To get them to that age, I take efforts to ensure a more slow and healthy growth rate. By 11 weeks they are around 6 to 7 pounds and by week 13, some are up to 10 pounds dressed. I think the flavor is excellent and I would rather butcher fewer, but larger, birds.
Under my feeding regime they would be too small to butcher at 6 weeks. By 8 weeks my guess is that they would be around 4 pounds dressed.
Good luck with everything. I really enjoy raising my meat birds. If given enough space and quality food, they are healthy, friendly birds who engage in normal chicken behaviors.