Hello Paul,
Thanks for your response. It sounds like we are pushing through the same issues. I also have jaw fatigue and some pain at times as I have had TMJ problems in the past. Also my regular embouchure has a slightly receded jaw setting so I am sure the jaw muscles are not used to the more forward set up when trying to stay in the “milk spot position”. I have found as you mentioned some success in this position as the air stream seems very focused and the slurs are easier, but like you have not been able to maintain it. I think your comment about strengthening is valid. I will focus on this and continue to work on the presto F and F# exercises. I would like stay in touch to share our experiences. Good luck.