Hello WhiteKnight
This is how I determine what Height ring I need for my guns. I mount the base to the gun and then I get the gun/base level and secure it in my padded vise. You could use sand bags or what ever to hold the gun for you. I then use Allen wrenches, you will need two sets, for example I lay two 1/4" wrenches across the base and then I lay the scope on top of the wrenches. If I need to go taller I use a larger size wrenches, or if I can go closer to the gun I will use smaller size wrenches.
I like to get the scope as close as I can to the gun. Once I know how close I can go I then measure one of the wrenches with a dial caliper. You can then go to the web-site of the ring manufacturer and find the ring heights from base to bottom of the scope. Get the size that is as close to your minimum height without going under.
I have been doing this for years and it has worked great for me. Several times I have been able to use low rings where the manufacturer recommended mediums.
Also there is a big difference in height sizes from one maker to another. One company's low might be .125" while another's low might be .250". I just pulled these numbers from the top of my head for an example.
Hope this helps!
GOOD SHOOTING!
Slufoot