Look up the specs on your scope. Find out how much total adjustment range it has.
If you have *lots* of adjustment range (70 MOA or more), you won't need any tilt in your base, you'll be able to go all the way from 100y/m to 900m.
If you have a small amount of adjustment (e.g. my 40mm Leup. 6.5-20X scope has 44 MOA total elevation travel, my B&L 6-24X Balvar has even less, I think it has a total of only 32MOA or so), then you *need* to have some tilt in your base in order to get to 900m. *BUT*, it is important you don't have *too much* tilt in your base, otherwise you won't be able to dial your scope low enough in order to shoot 300y/m (absolutely needed) or 100y/m (not strictly necessary, but it sure is nice if you can). In this scenario, you want to make sure that the tilt in your base is at least 5 MOA less than half your scope's total adjustment range. Using my Leupold as an example (44 MOA total elevation adjustment), half of 44MOA is 22 MOA, and 5 less than that is 17. So a 20 MOA rail might be just a bit too much, depending on how the various tolerances in your system stack. 15 MOA would be ideal, if available. You'd certainly need/want 10 MOA of tilt in your base.
Another possibility, instead of using a tilted base, is to use one of the Burris rings that use plastic spherical bushing inserts, which are available in a variety of offsets (the plastic bushings come in "0" offset, "5", "10", "15", "20" etc). With these, you use your ring inserts to point your scope tube where you want it to be.
