Calculating sight height
Measure the outside diameter of the barrel where the front sight will be. Lets say its .800". Divide this by 2 to give you the distance from the outside of the barrel to the center of the bore. In this case the answer is .400". Measure the distance from the top of the barrel to the top of the front sight, lets say its .600". Add this to .400" and you have the front sight height above the center of the bore of 1.000". This is the maximum height above the center of the bore that the rear sight can be when adjusted all the way down.
Now measure the barrel where the rear sight will be located. Lets say it is 1.500". Divide that by 2 giving the center of bore measurement. That comes out to .750". Subtract that from the front sight height above bore of 1.000" and you get a height of .250" that the top of the rear sight, when adjusted to its lowest position, can be above the top of the barrel .
You'll have to play around with different height ramps, sight blades, and rear sight models to get the right combination. Brownells catalog gives you the the necessary specs for the sights, ramps and blades, so you can order what you need.
Edited to add: Be careful when calculating the front sight blade/ramp combination height . The height that they specify for the front sight blade is measured from the top of the sight to the bottom of the dovetail. The dovetail section is about .100". A blade that they specify as .312" high is actually .212" above the ramp.
