Well, data is a bit inconsistant - you look for X 6.5 while no data for 6. Also for Y and X = 15.5, while no input data for 16.
you can change formulas as you complete missing data.
at the moment - I see no need to create custom functions in VBA - you can calculate this easily in the worksheet - see attached, there are only 3 formulas:
in P6 (I prepared intermediate stage table headers with X 6.5, 7.5 etc in column O and row 5) - your step1:
copied down and right
then in P14 (and copied right) your "step 2":
and finally in Q16
array* formula:
not in P16 because you gave no coefficient for X=6.5 (coeff. table starts from 7.0-8.0)
*...confirmed by pressing CTRL+SHIFT+ENTER to activate the array,
not just ENTER. You will know the array is active when you see curly braces
{ } appear around your formula. If you do not CTRL+SHIFT+ENTER you will get an error or a clearly incorrect answer. Press F2 on that cell and try again.
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