How to Calibrate a Hygrometer?

Being new to this and reading stuff for a couple of weeks now, I wound up doing the salt test with my analog hygrometer that came with the Humi.

Somewhere I thought that i read that you only needed to do this for 6hrs or so, at least thats what I had in my ind when at the 7 hour mark I took it out of the bag and put it in the Humi.

Anyway, when i put it in the bag and sealed it it was reading approx. 60% it slowly went up to a reading of 74% by approx. hour 3 and stayed there until i took it out of th bag at hour 7...do you folks think that is acceptable or should i start over and do the entire 24hr cycle?