Interpolation - quadratic

Hi,

I am trying to interpolate AGDD/x value for a given NDVI/y value using formula taken from the following thread on this forum

http://www.excelforum.com/excel-gene...rpolation.html.

My NDVI/y as a function of AGDD/x data has a quadratic shape so y is not (should not be) sorted ascending while the formula I think demand that order), but x is in an ascending order. In many of my spreadsheets, the result returned valid, but not always. I have uploaded a sample spreadsheet with this post. I will use this formula across sheets (I can do that), but as a second question, I need to return the results across sheets in to a new sheet by sheet name. Any help?

Sorry I don't have knowledge of programing.

Thanks,
Abys