Calculate the centre of the Ball and subtract the cutter radius off the Z as BillyStein recommended.
Y = (0.25 + 0.125) * Sin(Angle) - 0.25
Z = (0.25 + 0.125) * Cos(Angle) - (0.25 + 0.125)
Angle = 0 deg to 90deg. This will give a even cusp-height finish.
This
VB program should help.