I use a cabinet scraper to put a slight concave curve on the bottom of my bridges, and just trial fit it in place on the uke top. When close I draw witness lines on the bottom of the bridge, and then place a piece of 120 - 150 grit sandpaper on the top and slide the bridge bland back and forth over the area that it will sit. Useing the top as the sanding block so to speak. Very easy to get an exact fit,
Reason I use the top itself is that with thin soundboards it's real easy for them to take on more or less dome depending on the relative humidity when the instrument was assembled, the way it was clamped up or even the RH on the day you are glueing on the bridge. Perhaps small differences, but they will most likely be evident more often than not.