The other spreadsheet finds a few points on the curve to calculate "W" speed.
Let me know if you want the spreadsheets. http://www.apug.org/forums/forum37/8...3g-speeds.html
You would be free to experiment with them to make the needed changes to do a proper contrast index test, assuming this type of nomenclature makes any sense to you:
PS: your links above reminded me that I have changed my viewpoint on curve fitting. As may not be evident, my spreadsheets for EI and contrast calculations DON"T curve fit. I use simple triangulation to interpolate between the dots. That simple compromise has saved years of my life worrying about solving polynomial film curve equations for unknown variables and allowed me to take more photographs