Reading the inside of a tube with digital calipers is very difficult. For one thing, the jaws on the calipers only allow you to read the first quarter inch or so, essentially, the opening. Which could be out of round or pinched together, from the clamp ears. Unreliable, at best.
You need to measure down below that, inside the tube and inch or two, and for that, you need other tools. Like a machinist's hole gauge.
But I would either stop by a shop that has a graduated seat post measuring tool, or buy one if you think you'll use it often enough.