Here's a tip/trick for ya that I picked up from someone else who picked it up from one of the locals (I believe it was one of the Chu brothers, Will or Kevin) @ Connaught a few years back. If you haven't shot there before, Connaught is a wonderful range, with lots to recommend it. One of the downsides is that the entire range is a big field of grass, and the mounds are most definitely rounded / have a crown to them. Enough so that you end up with your bipod (for F-TR) on one side of the crest, with the feet pointing down hill (if they pivot like that) and your rear bag on the other side, pointing up hill. And thats on top of the usual uneven features and 'holes' you encounter on grass firing lines - it's not always possible to just 'move over', especially when shooting two or even three to a mound. Makes for some very interesting times trying to get the gun to track properly.
Anywho, the 'trick' that was passed along to me was to carry a few extra of the Edgewood 'donuts' in the range bag, filled only partway full. That way, you can shake 'em, squeeze 'em, or do whatever you need to get your rear bag sitting on a 'level' surface. Might have to 'pound' the stock/rear bag down a little to settle things, or else they'll settle on you during the string - DAMHIKT.
Works great on grass ranges, and also on ranges like what you describe - my 'home' 600yd range is like that, where the targets are at a slight up angle from the firing line.