Here and here you use the stock property PhysicsGlobals.KpaToAtmospheres

This property has the problem that it uses the CelestialBody.atmospherePressureSeaLevel of the home planet as definition of 1 atm

Usually this is fine, however if you try changing Kerbin's pressureASL then you end up having wrong ISP / deltaV calculations.

I suggest replacing all instances of PhysicsGlobals.KpaToAtmospheres with either 1 / 101.324996948242 or 0.00986923296440672

let me know if you want me to submit a PR