I add cracked corn to my 21% broiler starter to adjust the percentage of protein down over the course of their 8 weeks. Feed corn is around 8-9% protein so it's a simple math equation based on something called the Pearson's square. I use a calculator that I made in excel to calculate feed amounts and mix ratios, but there is a free website to calculate protein percentage in a mixed feed: http://homesteadapps.com/app/free/feedcalc/pearsonsquare.php
You wouldn't want to ever drop the protein level below about 16%.
I use this schedule:
First 4 weeks - 21%
Week 5 and 6 - 18%
Week 7 and 8 - 16%
Edit to add that the "concentrate" line in the protein calculator would be your corn and the "grain" line would be your feed grain