The measurement starts from zero point, you do not need to plus or subtract 10 mm or something like that, just measure the external and internal dimensions.
The minimum size for measurement is 20 mm, the maximum depends on the ruler.
This caliper is pretty accurate when printed on a photopolymer 3d printer. I used Anycubic Photon S. But there is still a measurement error, so use this tool for an approximate measurement. I have an error of about 0.5 mm, this is enough to measure