David Mercier
Ansys Employee

In the case of dry friction, the COF should be higher than the ratio Ft/Fn, with Ft = frictionnal or transversal force and Fn the normal load. The question is do you have a constant normal load in your simulation? Moreover are you looking for the COF during static or kinetic friction? The COF during the kinetic phase should be more "stable" rather than the one calculated during static friction (start of the motion).
The optimisation process is a very good approach. But why not trying your simulation with a constant normal load and a constant transveral load, then measuring slip distance as a function of COF (from frictionless to 0.5) and see when you have no motion. You should thus estimate a lower limit for the COF in your case study.
Here an interesting AIC about this question: https://courses.ansys.com/index.php/courses/contact-mechanics/lessons/frictional-contact-lesson-2/