As far as I know, there are four steps for recommending the N fertilizer rates based on NDVI readings and grain yield at N rich strip and farmer practice plot.

The steps are as follows:

1- Estimating Response Index (RI) RI=Ypn/Ypo

2- Estimating Ypo (yield at farmer practice strip)

3- Estimating Ypn (yield at N rich strip) based on RI, i.e., Ypn=Ypo*RI

4- N fertilizer rate recommendation using the formula:

NFRate=(N uptake at Ypn - N uptake at Ypo)/NUE

If the above-mentioned steps are correct, I would like to know to estimate Ypo what variables I should use as an independent variable? NDVIo (NDVI at farmer practice) or INSEY (In-season Estimated Yield based on NDVI, i.e., NDVI divided by days from planting to sensing)?