Hygrometer salt calibration test - should 2 TBSP table salt plus 1/2 TBSP water in a small monkey dish, in a gallon bag work?

this is driving me pretty mad, i've got hygrometers, digital and analog all reading wildly different percentages, and would like to calibrate them, read up on the salt test, where you take salt make it damp stick it in a plastic bag and after +8 hrs the internal humidity of the bag should be exactly 75%. for anyone that's familiar with calibrating hygrometers, should 2 TBSP table salt plus 1/2 TBSP water in a small monkey dish, in a gallon bag work? or is it too much water for the salt, too big of a bag, too much or too little time. did the test overnight on 4 different hygrometers for 12 hours, took them out of the bag, and now after 30 minutes to equalize to room temp, are not only once again wildly off, but the lowest is reading what I would imagine to be 10 percentage points of how humid the room "feels". maddening. thanks.