L#cky Strike
If I think I know what you're trying to do you'll want to use the EXCEL functions "MATCH", "INDEX" and "TREND" to perform linear extrapolation. What variables (and how many) are you dealing with? I assume 3; OAT, Hp and AuM and what format is the data in? i.e. is it columns of AuM varying with Hp for a fixed OAT etc?
I've used this method on a 3 variable calculation - Ng acceleration for a given Hp and OAT and it works fine.
LHB