I've measured a few frames for seatposts. Calipers have worked -- either poorly or not at all. The reason is that, if I'm resorting to measuring the frame, it's often because I also have a reason not to trust the diameter at the top of the tube. I've had good results with an inside spring caliper for going a couple inches down into the post and see if the diameter is uniform before making the final call. Getting within 0.2 mm takes some practice, and there's a good reason why machinists don't use those tools any more if they can avoid it.
An error of 0.2 mm in diameter would be 0.6 mm in circumference -- enough to notice in deformation of the slot.