Dawson Precision has the formula on their website, if you take a 5" average (4-6" low) at 25', your front site needs to be .111 shorter on on a 5" pistol or .128" on a 6" pistol.
You must have a tall front sight, my 2011's and 1911's are all running a .180" front sight.
Measure yours and subtract = the sight you need.