Stallie,
If you don't get any joy, send me a copy of the POH pages and I'll point you in a simple direction toward your goalposts. Rather do it with the data in front of me otherwise we'll go around in circles. A bit busy for the next few weeks but certainly could look at it after Christmas.
Yours is a typical problem I am faced with regularly.
The simple statistical method of running a multivariate regression tends to give more trouble than it's worth as the desired accuracy is paramount and the usually seen lumps and bumps in the carpet graph probably would make F(W,Hp) more confusing than useful.
A much easier way to keep the accuracy to whatever level you desire - while keeping the polynomial orders under control - at the expense of a bit more
VB coding - is to run the carpet as a set of individual 2D equations and then use an interpolation routine to get the final answer for the parametric variable.
Easy to set up and works fine in Excel.
Suggest you might have a look at Curve Expert - available via the net for a nominal amount and gives you more regression choices than a young chap should even dream about ....
JT