pressuretrols

as a heating professional, and fairly well versed in steam,  i don't know why i have such a hard time with this.  I *think* honeywell changed its "wording" sometime ago and maybe that is what is confusing me.  anyway, here is my question.  the old 404a's,  i used to set to .5psi and set the wheel to 1.  I never really knew what it meant, but it always worked residentially.  now i've been using the 404F's and those settings don't seem to work as well.  can someone tell me in a simple sentence or two, a good way to remember how to adjust this thing.  what does the main scale do if it is set to 1 psi for example and what would the differential scale do if it was set to 1.5 lets say for arguements sake.  the differential is now subtractive and i don't think it always was.  either way, i have a hard time understanding these things...