Maestro Thank you for your feedback!
For calculating a tuning curve, pianoscope does not use the measured partials directly but instead creates an inharmonicity model from which the expected partial offsets are derived. The percentage you mentioned controls the mix ratio of the modelled values with the directly measured ones.
The modeling has two benefits:
1) It can provide IH estimations for notes which have not been measured, which can save time.
2) The mixing of real values with the modeled ones seems to have a smoothing effect on the tuning curve which seems to limit the overall measuring error. I am not totally sure why this is the case, but experience shows that it does.
The value of 85% is a good default which you usually don't have to change to get good results. There are situations in which it can be beneficial to increase the value to even 100%. Namely if you encounter negative inharmonicity values for bass strings, which is quite rare. It experienced it some older upright pianos and on some Yamaha grands. The logarithmic inharmonicity model is currently not perfect at fitting these cases so that switching to a 100% can improve the outcome. That said, even in those cases many tunings were fine with the default of 85%.
I hope this helps.