After my first attempt to conduct this film test (where there was way too much flare in my test setup) I have now repeated and remeasured the 5 negatives and filled out the spreadsheet again.
In summary I think I have decent results. I have some questions about the toe area of the film. Contrary to my first effort where for some reason I didn't give the step wedge sufficient exposure and ended up with a long toe and low Dmax, this time time I have barely any toe present !! [1]

My plan now is to replace some of the points say from 28-32 with a toe interpolated between one more reading at D=3.8 (see below) (I will also need to space out the y-axis values over that region too (because the default step is 0.1 and I will need it to be about 0.2).

I think this points to a flaw in the spreadsheet as it assumes you have sufficient toe present in the data.

Anyway, any comments on my plan and spreadsheet would be appreciated.

[1] This lack of toe is despite me bracketing from a nominal exposure by metering the mid grey step to 3 stops below. I also 'nominally' rated the HP5+ at 200ASA. If I had rated it at 400ASA and bracketed +/- 1 stop I would prob. have sufficient toe. Fortunately I also included an X-rite cal step wedge which has its darkest step at D=3.8 (the Stouffer 3 only goes to D=3.05). That permitted me to get ONE point in the toe region and I can then interpolate/guess the others between 3.05-3.8