Which IOR scope? If your recticle is in the 2nd focal plane and your are using it for range estimation, this will only work at a set magnification. I have an IOR 6-24x50: to use the scope for mil estimation it must be at 12x mag, for MOA it needs to be at 18x mag. (this might vary with individual scopes and would need to be checked at 100m or yards as appropriate - but it is spot on for mine). However, the turret clicks will always be a 1/4 MOA regardless of mag setting or range! Basically, 1 click = 1/4" or 6mm at 100yds/100m. For a scope where the recticle is in the first focal plane, it will always give the true value.
To use mils: I mil is exactly 10cm at 100m, 20cm at 200m , 30cm at 300m, etc. Therefore, to estimate range on a Roe using mils: average Roe adult is 60cm from chest to rear, so at 100m the Roe will subtend 6 mils, 3 mils at 200m, 2 mil at 300m, etc (with the IOR, the mag must be at 12x). For bullet drop compensation, 10cm is just about 4" - so, for 4" drop at 200m this will be 1/2 mil, and 8" drop at 300m will be 2/3 mil (again for the IOR at 12x mag).
In the end, it is what you are used to, you don't have time to start mental maths when your finger is on the trigger and the deer is about to run!
Bryan