Looks to be very useful. Unfortunately I can't get the Frequency slider to move. What am I doing wrong please?

Seems to work OK for me. I will point out that it is very "coarse" and only has 4 possible settings. Perhaps not the best choice for a slider, but perhaps they made that UI choice just to match the other sliders?

Strictly you can NOT actually get perfect models with 100% prediction for any coil design because magnetic fields can only be solved directly with Maxwell's rather than the simple algebraic equations being used by this app. That's generally OK because getting close is more important than perfect. You ALWAYS must physically build things like coils and validate them empirically by testing if you need "perfect". There is no other way (even using a 3D FEA using Maxwell's equations won't give you perfect either).

I think it is still a work in progress.

Very nice project. Much better than dealing with the raw tables/curves from Fair-Rite which seem to have been photocopied again and again over the ages.