Most ring suppliers provide the height from the top of the rail to the bottom of the scope tube. Once you identify the cheap ring that gives the correct height, measure the ring height with a caliper or accurate ruler.
Scope height is not the only parameter that determines the best cheek weld when shooting. Other factors bear as well, like your skeleton, clothing, shooting position, stock length of pull, scope eye relief, etc. For example, it's not uncommon for people to use the next taller ring to provide more objective bell clearance and allow them to move the scope back further to optimize eye relief.
Extra time spent dialing in your scope position will pay off when shooting later. Thoroughly evaluate various cheek rest positions using the cheap rings. Of course, you should do this in the shooting position you use most often. Wear the same clothing too. When you think you have it right, close your eyes, establish a comfortable cheek weld, and then open your eyes. Is the sight picture correct? If not, keep playing with scope position and cheek rest height until it is.
Last edited by bruce_ventura; 08-03-2012 at 09:30 AM.