I have a data set with that I want to expand from 5 numbers to fifty numbers. The five known numbers represent positions 5, 15, 25, 35, and 45 in the set.
x y
5 400
15 500
25 700
35 1000
45 1400

I know I can get a polynomial best fit equation to expand this to cover all of 1-50, (in this case y = 0.00833333x^3 %2B 0.0178571x^2 %2B 7.64881x %2B 358.839) but if any of the 5 original numbers changes then the equation must be recreated. The five numbers are the product of another formula elsewhere in XLS and so can be changed frequently. Is there any way to create a polynomial regression type formula automatically so that no matter what I do to those 5 numbers my 1-50 can be recalculated automatically?

Completely lost on this one, any help would be hugely appreciated! Thanks.