Typically I use the original gamma ray logs, along with shale and clean endpoints defined for each zone, within each well. I then calculate volume shale using the appropriate equation (linear or non-linear). This approach creates a volume shale result which can be compared well to well, because individual clean and shale lines have been selected for each zone, within each well, eliminating the differences between the gamma ray logs.
The other way that this can be done is to first normalize the GR logs, based on a high reading zone and a low reading zone, and then use the normalized GR logs, along with constant shale and clean endpoint values to calculate volume shale. I have used both approaches over the years and prefer the former to the latter to create a consistent volume shale output.