Yes, I fear that will be a difficult decision to make. Yes, it will look flat, so stop before that. I think you'll need to be careful on the toes, and the part of the foot closer to the toes - that will start to look flat first. On the heel, you should be able to go more aggressively.
Well, it was guesswork, but yes, with experience mixed in.
Because Levels are always better than Curves.
It might also be possible to do it with the Handyman Method, but more time-consuming I expect.