Resetting the gauge face to 0 or 100 does essentially nothing to actually calibrate or verify the gauge. There needs to be a measurement of force somewhere (besides 0) in the stroke. Setting the full stroke without measurement force means the slope could be anywhere from 0 lb/in to infinity. The ASTM standard requires measurement with known blocks throughout the range:
https://www.msi-viking.com/Mitutoyo-64AAA964-Durometer-Test-Block-Set-Shore-A-Scale_p_23530.html
A cheaper method is to be sure to keep the durometer clean, avoid impacts, and compare to another regularly.