Modifying a double interpolation function into interpolation/extrapolation function

Hi

An earlier thread was posted by me in another forum (see thread: http://www.ozgrid.com/forum/showthread.php?t=69973). In that thread, I got a usefull macro (see Post# 13) that enabled me to solve the problem explained in Post# 11. The problem can be explained as follows;
I got a table with three variables (2 independant and 1 depandant). X and Y are the indep. and Z is the depend. Z values can only be obtained if the intercept between an X value and Y value is known. If I have an X value and a Y value and I need to find the Z value then I need to do two interpolations; one between X values and Z values and the 2nd between X values and Z values. An answer on how I can do two interpolations to find Z values (C15:C25) in the second table when the X values (A15:A25) and Y values (B15:B25) are known was provided in Post#13 of the mentioned thread. However, my enquiry now is how to modify that interpolation macro to do extrapolation to X and Y values that fall beyond their tabulated values. When I used the above macro, that was intended to be only used for interpolation, to extend it for values outside the table, I got this msg.: "Value not within table extent" (see table No.2 in the attachment).

In that thread, my question was to find an interpolation solution and that what i got and this time i am after a modification to that useful macro.

Abdulrahim