If you get a set of the Burris rings that take the inserts, you will have to purchase a seperate box that contains the 20 MOA set of inserts or you will be limited to the ones that come with the rings. I think they come with the 0 MOA, 5 MOA, and 10 MOA inserts is all. But if you buy the 20 MOA set (I forget the part #), you can use them in rear, and the 10 MOA set in the front and get a total of 30 MOA slant. Place the -10 insert on the bottom in the front ring, +10 on top, then the +20 on the bottom of the rear ring, and -20 on top of that.
If you've never used these before, they're just a set of inserts with each set containing a + and a - of the same number. The +10 for example indicates that it is approximately .010" thicker than a "0" insert. So to keep the hole the same diameter for the scope, you must use the matching -10 insert, which is about .010" thinner. Same with the other inserts.
You want to cant the scope forward using the rings so you don't have to use the internal adjustment to do so, so the thickest of the inserts (+20 MOA) would go on the bottom in the rear, and ideally, you could use another set of 20 MOA in the front with the -20 MOA on the bottom there... the -10 MOA on bottom up front will probably be about right in your case. You may just start out using the +10 MOA on bottom in the rear and a -5 MOA on bottom in front for a total of 15 MOA, and you might easily get the adjustment you need from that without even buying the additional 20 MOA insert set.