Thread: Picatinny rail modification.

Over the weekend we were down Tekapo shooting long range gongs. It was discovered at just over 1250m my brothers scope ran out of elevation. On investigating the turret there was with a 100m zero 35moa below the zero. No wonder the top end was limited. He is running a 20moa picatinny so needs a 40moa which are not common for a Tikka T3 Varmint. So he will probably get the rail machined. The question is with the rail at 139.50mm long how much should be removed from the front to get another 20moa?
Thanks to those with different brain to me.
Jeremy