Here is the thread:
http://www.halowars.com/forums/1/539611/ShowThread.aspx
Timotron says:
So this has come up enough that I went back through my notes to find the supply rate formula
S = supply rate of base building, N = number of supply pads total, Sm = modified supply rate for a warehouse or supply pad
Sm = (S * 1.5) / (((N / (13)) + 1)
Base supply rate (S) is 2.5 for a normal supply pad/ warehouse
S is 3.5 for a heavy pad.
In the build order template from this thread:
http://www.halowars.com/forums/1/580162/ShowThread.aspx
The diminishing returns formula and table for supply pads is in the last tab of the spreadsheet.