The SHGC data is measured data
and not based on a mathematical function.
I copied four of 11822 measurements of
different types of glazing from
http://windows.lbl.gov/software/window/window.html
If all the data points to 3 significant digits are used
and fitted to the Gompertz function,
the fit is extremely good.
R-squared= 0.999999
Here is the updated file:
http://www.mediafire.com/file/wmdjmlygmmw/03_28_09.xlsx