I know that it seems to be very unscientific to just settle on 1 gallon per thousand for any and all backpack sprayers!! Definitely variables such as pump pressure, nozzle output etc etc come into play, but, over the years I've never had a bad experience using that rate as a guide. However, calibration is still recommended on occasion to check. I'm currently using a battery operated Shurflow sprayer that has four adjustable pump speeds so I have calibrated those out. Sounds like your calibration methods and numbers are spot on!