A US DOT-3AA steel cylinder may be filled to 110 % of its marked service pressure, the famous plus sign after the test date, but only if its wall is thick enough. The rules accept two ways to show that: the thinnest wall measured with an ultrasonic gauge, or the elastic expansion at the hydrostatic test compared with the cylinder’s rejection elastic expansion (REE). The calculator does both, and works out the K-factor from your measurements if you don’t have it.
It covers the wall-stress part of the rule only. The visual inspection and everything else still have to pass on their own.