Simplistically: 3 clicks on an MilRAd scope to 1 MOA. Or 1 click per 1/3 MOA (ish). Works for field expedient conditions at least.
I actually prefer this to 1/4 MOA click scopes, as in my experience 1/4 MOA doesn't move you enough to really matter, and 1/2 MOA can be too much. 1/3 MOA (or 1 click on a MRAD scope), is juuuussst right.