I use a bore gauge myself (with a micrometer, obviously), to get down below the slit of the seat tube clamp. As many have mentioned above, the area of the slit may well have been distorted due to prior abuse, and in that case will not yield a correct reading of the seattube ID.
Before measuring with a bore gauge, you want to make sure you're measuring the ID of the tube and not crud/or corrosion that has built up inside.
If that requires cleaning out the seat tube (e.g. with solvent & rags, fine sandpaper, or a brake cylinder hone), you probably want to do that with a bare frame and not let the removed detritus descend into and contaminate the assembled bottom bracket.
Nuff said?