Using conditions given in the sketch above, let's do a parameter study.
M = AsFy(d - a/2)
a = Asfy/0.85fc'b, note, since As, b, fc' and fy are constant throughout this exercise, "a" is constant as well. Let's assume a = 50mm.
d = 65, M1 = AsFy(65-50/2) = 40Asfy
d = 115, M2 = AsFy(115-50/2) = 90Asfy
d = 165, M3 = AsFy(165-50/2) = 140Asfy
M2/M1 = ...