There has been an addition to the spreadsheet, explained in the post:
Back in the day, when I was building a pricing model for mortgages that would be sold to FNMA, I ran into a problem with their pricing tables. The table below shows the problem. The X column is the dollar price the agency would pay, at various yield levels (column y). Since we were underwriting new mortgages, we needed the yield for a par price (100), but it was not on the table.
Since we were offering over a dozen or more different mortgages, we had a dozen or more tables to find the par price each day to set the days mortgage yields.
I built the formula with the option of finding the interpolated value, if the X column is ascending or descending:
Of course the x value need not be 100.
Nowadays, the agency interpolates the par price and posts it on the table, but it may come in handy for other interpolations.
Look for “Interpolate”