OK, for those of you who didn't read the whole OP before responding - thanks for at least reading the title. Yes the unit has been weighed and I read the owners manual.
For the others with experience, your comments were appreciated as you understood that once the coachwork is on it is impossible to get a true starting point (measurement location) as suggested in the owners manual.
It appears that the starting point should be when the coach is level as determine by the interior floor. This may be 0 psi (or what ever levels the coach) with a new unit vs one with sagging tied springs.
Adding PSI from that point is a trial and test method until you find a setting you like - ride wise