^^^
This, if you've got the dough. I made my own stock with one, but if you're handy around wood/fiberglass, you can modify many stocks for this.
One of the most popular (and inexpensive) ways to get an adjustable cheekpiece is with the Karsten:
http://www.tacticalworks.com/Karsten...t-A-Model.html
Depending on how much you need, a padded stock pack, or cheek pad is even cheaper, can hold ammo and other junk, and you don't need to drill your stocK:
http://www.tacticalworks.com/Tactica...Cheek-Pad.html
You should use the lowest height rings that will allow the front bell of the scope to just clear the barrel. This obviously is what minimizes the height of the comb to get the proper cheekweld and eye relief.
It's a function of the ring heights, the diameter (or really, half the diameter) of the front bell housing, the length of the optic and how you have it positioned on the rifle, and the diameter/taper of the barrel... a lot of variables.