At what point does a 20 MOA base become necessary?
When you run out of adjustment in your scope for shooting at longer distance. A scope with a lot of adjustment range (like a 30mm tube for example) is less likely to need a 20 moa base than say a 1" tube with less adjustment range.
Any way to determine this before you build?
Know the trajectory of the round you'll be shooting.
Know the distance you want to shoot.
Know the adjustment range of the scope you will use.
Take into account you may use up some of the available adjustment sighting in at your zero distance.
