How to figure out taper for a scope base , adding MOA

[James Jones]

I'm trying to desgine a scope base for my AR and need to know if their is a formula to get the MOA added to a scope base , being that the AR is flat how much higher in the rear will a 7" base need to be for say 20-25 extra minutes?

 

[Buffalobob]

ABINOK

Shim thickness inches = (MOA wanted X base lenght)/3600

Measure the base from the front of the base to middle of the rear screws