The image above shows how to calculate how deep to drill, using a No.4 center drill, to give a desired hole size.
The basic calculation is,
Drill Depth = 0.125 + (Desired Hole – .125) / (2*SquartRoot(3))
Or, Drill Depth = .125 + (Desired Hole – .125) / 3.4641
For 0.125 < Desired Hole < .3125
For desired hole sizes less that 0.125 use:
Drill Depth = (Desired Hole / 2) / Tan(59deg)
Or, Drill Depth = (Desired Hole / 2) / 1.6643