Originally Posted by dddd

For accurate sizing, you really want to test the size by clamping down on a post or a sizing rod, noting how much clamping movement is needed to secure the post.
The caliper won't measure easily when the seat tube opening or seat post (think steel post or one that's been well-used/abused) isn't quite round. You'll have to get the gauge tips aligned with the tube to get the biggest reading for seat tubes, and likely have to take an average of several readings, so far from the best way.
Those Chinese steel dial calipers seem to be much more than accurate enough if used for things that such a tool is designed for.