I'm currently kitting out my new 26" 300 win mag for long range (1000 yds.) shooting. The scope I'm looking at has 18 MIL (or 60.75 MOA) of total vertical adjustment. I'm wondering if I should get a 15 MOA base or 20 MOA base.

I would like to be able to have a 200 yard zero and still be able to track to 1000+ yards. Would the 20 MOA likely be too much for the 200 yd zero? Would the 15 MOA likely be not enough for 1000+?

I would appreciate any input.

No clue why you want to zero a rifle at 200 yards if your intentions are to dial the scope for elevation.

I put a 20 MOA rail on all of my rifles. Even my 300 WM that only needs 7 mils of correction at 1000 yards.

Here.

http://texashuntingforum.com/forum/ubbthreads.php/topics/6726169/1/How_much_canted_base_is_needed

Let's calculate it. You have, call it, 60 MOA of internal travel. With a zero MOA base, you should have 30 up and 30 down. Add a 20 moa base and you add 20 moa up to your elevation, but lose 20 moa on the down. So your reticle is positioned with 50 moa up (30 + 20), and only 10 moa down. My rule of thumb is never go less than 10 moa on the down side. When calculating your exact available moa travel, often times the tolerances between the rings, bases, action and scope are not 100% exact, so you have to allow for some tolerances. Mine is 10 moa on the down side. Theoretically, a 20 moa should work, but you are right at the very edge of it working properly.

If you were to get a 10 moa base, you would have 40 moa up and 20 moa down, which would be fine. I would go 10 moa base to allow for more cushion to make sure things work properly.

Depending on which bullet you are shooting, even a zero moa base should work. A 208 A-max at 2900 fps only needs about 26 moa to 1K yards. You should be good out to 1100 yards dialing, plus any holds using the reticle.

This is a good example of why you might want a 15/20 MOA base. You are dialing in 25.5 MOA + whatever MOA needed to get your 100 yd zero (going to assume +3 MOA for sake of argument). This give you a total of 28.5 MOA.
If your scope only has 60 MOA of travel, (30 up + 30 down), you are very close to the top of your scope's adjustment.