pressure is definitely not linear if it were then increasing the load from:
1 1/8 1,145 Cheddite Green Dot RXP 12 20.5 7,100
to:
1 1/8 1,200 Cheddite Green Dot RXP 12 22 7,800
Pressure would rise to 7620, not 7800. At the same rate the 22.3 would give 7725. I suspect the curve is more or less parabolic with each increase of powder charging giving a greter pressure rise than the last. Even so you are starting a quite low pressure & only getting an increase of a bit over 1%. I seriously doubt the pressure will exceed 8,000.
Pressure can vary depending upon a lot of factors, but I believe shotshells are less influenced by outside factors than rifle cartridges with the pressure being primarily built into the shells them selves. Crimp strength can vary pressure to a greater amount than a tenth or so grain of powder charge.
