For the first ~2 weeks I let mine eat as much of the chick feed (usually 20-22%) as they want - 24 hours a day. Then the next week I start the 12 hours on 12 hours off thing with the chick feed. By about week 3 I had them on 18% protein broiler feed and they ate that until harvest day. No leg or heart issues. I think too much protein can make those issues worse. I found the 18% to be just fine for me and my birds were huge by the end of 8 weeks. Between 6.5 and 8.5 lbs, dressed.
Good luck with your birds. Don't stress too much about the details, but I would say that starting the 12 hours on 12 hours off by week 2 or 3 is the most important thing (in my limited experience).
EDIT: Wow, I'm sorry, I realize now you were asking about feed per chick per day. I didn't really answer that so my bad. I did not measure my feed like that because I think it would have resulted in some chicks being "starved out" by the more aggressive chicks. However, I did use the chart on
this page to help me gauge how much feed I would need to buy. Scroll down to "Estimated Growth Rate and Feed Consumption of Jumbo White Cornish Cross Boilers". In fact, I used this whole page as a guide when I raised my first Cornish X. It was helpful.