Since this topic comes up every so often I thought I would document the method I use. Lay down three, 2" wide, strips of masking tape and locate bridge location. I commonly use 25.4" scale length (+ .1" compensation) and this is measured from the nut to the center of the saddle on the 1st string (little E). I then subtract 1/4" as this is the length from the front edge of my bridge to the saddle centerline on the little E. I then use this mark to locate the front edge of the bridge.
Next I put a mark on each end of the fingerboard in the exact center and then draw a line from these two fingerboard marks onto the masking tape. This will now be the centerline of the bridge location. Next measure the bridge (between the outer E sting holes and draw a centerline mark on the front edge of the bridge. This mark will then line up with the centerline of the fingerboard. Next I take two drafting squares and draw a perpendicular line from the centerline to the leading edge of the bridge. This assures that the bridge is now perfectly square to the centerline.
Now place the bridge on the line / leading edge line and hold it down firmly or clamp it in postion. Drill holes through the outer E strnig holes. Clip off the heads of a couple of plastic bridge pins and insert the pins into the outer E string holes and this will now hold the bridge in position. Draw around the perimeter of the bridge and remove the bridge.
Now draw a line 1/32" inside of the traced outline of the bridge. Take a razor and cut the tape on the inside line and remove the tape. This is the area that I will remove the finish.
I use a Dremel, router base and 1/8" carbide down-cut bit and make a test cut begining at one of the holes. After the correct depth is set (just to remove the finish and no wood) rout off the finish up to the masking tape.
Once the finish is removed use 180 grit sandpaper and clean up any fuxx left in the gluing area.
Remove the masking tape.
Test fit the bridge before gluing.
Apply glue (I use LMI or hide) to the bottom of the bridge and to the top and let it tack up for a minute or so if using white LMI or slap it down quick if using hide glue.
Clamp brige in place and clean out all glue squeeze out. I use a UHMM plastic caul inside to back up the clamp on the bridge plate.
After the glue is thoroughly dry (24 hours) remove the clamp and tap the bridge pins out (from the inside out).
Although my bridge does rest on a 1/32" perimeter of finish I have not noticed any negative tonal (gasket) effects in the energy transmission. I have removed bridges and the finish (.003" - .005" mils thick) actually compresses and the there is wood to wood contact in the periphericy of the joint with no visible glue build up. I have remove the finish [in the past] to the exact outline of the bridge but it is a lot more difficult to get a clean looking fit and now prefer this method.
Hope this helps and answers some questions that you may have. There are many other ways of doing this step but this is the method that I have settled on, for now...
It definitely needs to be added to the jigs and methods area. I'll definitely give this method a try.