Niek Otten wrote:
One way (not necessarily "the best" way) is to use this UDF. Instructions for UDFs at the end.

I have written and used similar functions, but they can become very slow
in large files, especially when automatic recalculation sort of gets
lost and recalculates every thing every time whether it needs it nor
not. I think functions written in the modules are far far slower than
built in functions.

An alternative is to use MATCH to find your table entry row then use
INDEX to pick out the four values you need for interpolation.

John