If your springs flex much at all, then the usual limiting factor you would use to set the height of a bump stop would be the limit of the shock compression. If you have more than 1.5" of shock compression left then you can make that bump stop shorter. The bump stop should just hit before the shock bottoms out so you don't tear up your shocks.
In the back, the shocks are set at an angle. I guess it is about 30 degrees from vertical. So since the axle travel is almost vertical, when the axle travels straight up to the bump stop, the shock will not compress as far as the axle travels. The shock will only compress at a rate of the cosine of the angle from vertical. For 30 degrees that is about 0.87.
So if you have 2" between the axle and the bump stop in the back. The shock will compress about 0.87 times that amount (0.87 x 2 = 1.74) or about 1 and 3/4".
In the front the shocks are pretty much vertical so the axle travel and shock travel under compression is about 1 to 1 or the same if you want to think of it that way.
So if you change the springs, make sure the bump stop hits before your shock hits bottom.