I have a scope I need to adjust.

Pretend the cross hairs don't move.

If the optic currently points too high (meaning the bullets hits low) and the rings are 5" apart.

How much would I have to shim up the rear scope ring to make it point 24" LOWER at 50 yds?

Is there a calculation for this?

Whatever you are willing to put up with, is exactly what you will have.

When your ship comes in. ... make sure you are willing to unload it.

PAYPAL, sucks and I will never use them again. I recommend you do the same.