I have a Mark IV with a 50mm objective and I'm using medium rings with a 20 MOA base. I have plenty of clearance between the scope and base. The only thing you gain with taller rings is you can mount the scope farther back. With lower rings you have to move the scope forward so the tapered part of the scope (where the body transitions into the objective) doesn't hit your base. You should have at least 80 MOA of adjustment in your scope, which gives you 40 up and 40 down. I run almost 40 MOA in my 308 to get to 1000 yards. If you don't plan on going past 1000 a 0 MOA base will work for you. But if you decide to buy a one piece base it wont hurt to get a tapered base because you'll never use your 40 MOA down adjustment and you will have the option to shoot farther as you gain experience. Midway had a sale on bases you may want to check them out, the ones that caught my eye were 35 MOA bases for 50 bucks.