The thread title talks about interpolation, but most of the algorithm appears to be using a regression type of algorithm. After regressing parameters in J48:L52, you then calculate Y at each PA, then a linear regression algorithm (FORECAST.LINEAR() and TREND() both use the same basic linear regression algorithms) to find the final value.

If you want to do linear interpolation on the values in N48:O52 to get the final value and are not limited to Excel as a spreadsheet, Gnumeric is a spreadsheet (native only to Linux OS) that has a built in INTERPOLATION() function that makes short work of linear (an other) interpolation problems.

If you must use Excel and you really want more of an interpolation algorithm, my preferred approach to setting up an interpolation problem in Excel: https://www.excelforum.com/excel-for...ml#post5262186

If I'm understanding what you are trying to do, see if any of that helps. If I'm not understanding, help us understand what you are trying to do and the algorithm you want to use to do it, and we should be able to help you program that algorithm into the spreadsheet.