Hi Marc,
> Maybe a center radius point that could be draggable?
Right, but that's the one that requires a different solver.
Instead of solving the problem "find the points on the 2 curves to make a fillet arc of the given radius", it's more like "find the points on the 2 curves to make a fillet arc with the center closest to the given center point, with radius being calculated".
It's a different problem to solve and would require a different solver mechanism to be written to calculate it  that's certainly possible but it's a fair amount of work to do it. So that could be possible in the future.
 Michael
