One particular standard caliber ammo choice of interest was about 1000fps as described. Then there was a +P selection listed as 1100 fps. So to me, +P adds 10%. Not sure what +P+ might be. Just as a guess- maybe another 10% above that? Making for 1210fps?
There is no
direct correlation between pressure and velocity. Numerous other variables come into play.
My guess is that the precise 10% velocity increase is a coincidence.