Checked my math with the guys over at snipersparadise, and the easy math is this.
The TAN of the angle you wish to achieve (20 MOA) multiplied by the distance between the scope mounts, center to center, (3.625) equals the distance the rear mount needs to come up (.021), to check this you can merely change the distance number to a yardage in inches ie 3600 for 100 yds which comes out to 20.94" or approximately 20 MOA.
But you have a Ruger Hawkeye and I don't know the exact center to center distance between your rings.
So what you need to do is measure the distance between the ring centers, and use this formula.
Tan(X)= (shim offset)/(distance between ring centers)
That means InverseTan(shim offset/distance between rings) = X
X is in degrees so multiply by 60 to turn it into minutes.
For example, InverseTan(0.010"/4") = 0.1432. 0.1432x60= 8.6 MOA
You can also play around with the formula to figure out how much shimming you need to get the MOA you want. Say you want 15 MOA, (15 minutes is one quarter of a degree, 15/60=0.25).
Tan(.25) = X/(distance between rings). Lets assume 5 inches this between rings. Tan(.25)x5"= 0.0218" of offset.
I hope this gives you the tools you need to achieve the effect you desire.