Others may have better answers . . .
The broilers grow so fast that you can alter the growth rate by choosing a different protein level in the feed.
You only have about 4-6 weeks when normally the chickens are butchered at 8-10 weeks. SO do a bit of research to determine just how fast you need these babies to grow. THe have a SUPER appetitie. THe faster they grow the more feed they want. YOu really need to be careful about over feeding as they can grow tooo fast and have health problems.
I liked how my Cornish crosses grew on fermented feed. IT is easy to do and because the feed is saturated with water they cannot gorge like eating dry feed. Please take a few minutes to research fermented feeds on BYC. Kuntry Girl did a nice experiment on raising them on fermented feed. Less odorous poo and fewer losses too.
My dual purpose birds were raised on 28% feed. But understand they have a lesser appetite than the Cornish meat birds. Meaning you need to consider the total protein in a day: a big eater eats more feed and that can translate into more grams of protein a day.
These birds get VERY hot also as their metabolism is running at a super rate.
Perhaps you could contact the source you are purchasing from for a recommendation.
( You can ferment any of the feeds, no matter the % protein in the crumble/pellet.)