Test results - take 2
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 !! 
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.
 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