Finally got round to studying the Excels.
Must commend .There are thousands of numbers! Your organisation and manipulation puts my efforts to shame.
Calibration I will leave to "measurement accuracy questioners".
Have no problems with your Wattage calculations other than puzzling over L=0.008m now appearing (rather than the original 0.01m).
As I understand you have set the 100% Efficiency as that at a Water Temp of 0c( W=Wo - 0.2635Tw)?
This seems somewhat arbitrary and needs justification - I am not happy.
However using the technique show trends is probably OK.
Maybe worth a recap to you 1st graph ,
here 
Although this describes the behaviour I would expect from a "Case-enclosed-MB", I do not think it is adequately described by constant "die-MB DT" and constant R.They indicate a change of heatsink temperature(case or room).
Got completely lost by your "poly" manipulations - however will probably dawn.
Just ramblings not a point scoring mission.