Chair seats are not only about legs forward, they are about thighs too high. THe best solution: time in two point to get all the joints of the legs elastic/stretching down and back with lowered heels. There are two things you have to identify: tension/ability to keep the stretch and knowing where your legs are. Yes, time on a lunge w/o stirrups/lengthening the leg (down/back/toes still up) can help. But just standing in two point and pushing leg forward until you sit down, and the pushing them back until you fall forward teaches you to know where the legs are. Time w/o stirrups can get proper placement (although the length should make no difference) because a lowered thigh/stretching from hip to heel slightly back should keep it remedied. Posting trot w/o stirrups will help because the rider can learn to release more, and it is not the pinching per se, but the lifting up (not inward) of the thigh/knee; that is why two point helps because it is about stretch.