Hi Yanchen,
you can have software like Matlab calculate where the smallest radius point is (arc length calculation), or use a visual indication like what you show to then eyeball it, which in my view works rather well if you zoom in sufficiently. The thing with setbacks like yours is that you want to get the best surface flow and as few isocurves as possible by minimising the corner area to be filled. Massaging control points is necessary to achieve G2 surface continuity (according to the tolerances you’re working towards, meaning what angle is acceptable depending on product, manufacturing method and of course size - you probably won’t bother with G2 if the setback corner is tiny in reality or if the material hides minimal discontinuities).
Creo or SolidWorks have a tool that patches these regions, like Rhino and Alias have, but when you analyse the former two’s results, they also often enjoy surface perturbations. This is where massaging control points can achieve what software still can’t always achieve, in my view. Ask technical surfacers working on class-A surfaces for aircraft or cars ; )