so that gives me all the info to calculate altitude given QNH the lapse rate and the PA from the encoder...
You don't need lapse rate to convert between altitude as ATC will see it. Nor do you need a formula like that. All you need is to know the pressure variation with altitude in the standard atmosphere to determine the offset for a particular QNH.
Bookworm - those of us in the terminal environment would rather use Altitude than Flight Levels as it makes life a lot easier.
That's only because you're assisting aircraft with objective A by assigning levels at or above the MVA.
