Not to make too much of an obvious point, but the relative heights of the front and rear rests is determined by stock design, and the angle of your line of sight.
I have an old SG&Y Mellinium that has butt that measures only 3 3/4 tall, and at the Visalia range we shoot slightly downhill. In order not to have to put a spacer under my Edgewood rear bag, and so that I would not be right on the edge of my Hart rest's lower elevation limit, I had the bottom of its base machined off flat with its center hub. This was worked out very well.
When looking at rests, I place close attention to how low the plate that the front bag sits on can be adjusted, looking for no more than 5" minimum height, with enough leveling point extension to be able to level the rest. More recently, I have had a custom Otto ring made for that rear bag, and I like the stability that it gives, as contrasted with other spacers that I have tried that seemed to degrade rear bag stability.
