Natchez supply has a chart that tells you what rings to purchase according to the size of your objective lens. Like stated above there are other factors that contribute such as cheek/head placement, etc.

Rule of thumb is as low as possible however like mentioned the ring height is both a personal and equipment based choice. Test to determine if you rings are to high is the same one that you use for setting eye relief. Close your eyes, shoulder your rifle to your natural/familar shooting position and then open your eyes. If you reach or duck to getthe sight pictureyour rings size isn't optimal for your positioning on that rifle. If you have never done this test before you may need to do it a few times to ensure your findings are accurate.

I'd suggest Burris Sig rings with the offset (you have to order these separate) inserts.These allow you to get as close as possible before you start making adjustments/chasing the bullet using the scopes turrets.