I use a FEL-PRO® BS13044-2 Replaces old# BS40012
I make a template about 4 inches long like shown that is tapered from 3.407 (this is the normal crank
size at the seal) to 3.300.
Trying to use a dial caliper or other measuring device is hard to center and pushes against the seal.
I mark the template where I want the seal I. D. to be (3.388) which gives me about .020 interference
fit on an unmodified crank seal area.
I have talked to several people, Ford, Gm and Felpro, with interference fits ranging from.005 to .025.
I have also measured Ford and Chevy seals and I came up with about .020 as an average.
I also graduate the lines on the template perpendicular to the length at several places.
I don't try to take the seal right to a size so that I don't go to far.
You have to put the block on its front so that the template slides in the seal area easy.
I put the seal in an old cap the way it goes and file it to about .075 protrusion on one side only
leaving one side untouched.
After filing the seal will be frayed somewhat so I cut the seal edge in a motion like a paper cutter  
with a new razor blade.
Put the modified seal in the block (standing on its front) and cap and measure, repeat till you get the
size you want.
Measure in cap
File seal
Cut seal
Measure in block