quark gave very good formula in post #230. However H is not (km) but (m)
from where (P in hp) we get height in feet H' =((44331.5-4946.624 x (P x 100)^0.190263)/0.3048
Itsjustme should have no problem with QNH.
First calcul H' for certain P in hp, then subtract from it next H' that...