I would only suggest that by fitting to the weight calculated from the actual count, you are adding to the theoretical error by rounding inherant in the calculations. In principal, I would first correct the actual measured data and then convert it into whatever units I wanted. In practice, I'm not convinced that it will really make a difference. You will have to try and see if you get close enough. As for the coefficients I gave you, they only got the "fit" into the neighborhood. I don't know how close you need it to be. If you are going to use an algorithm to generate the fit, you will need to be careful to understand what the algorithm is doing. You may need to add more decimal places to the coefficients to get the fit as tight as you seem to be suggesting. Also, did you understand what I was saying about using the spreadsheet to fit the curve manually? Try it. By watching the graph and the tabulated data change as you change the coefficients, it will make fitting the curve a bit more intuitive. If you'd like, I'll be happy to try and explain the process better than I did before.