Agreed. My understanding is that the symmetry of thrust at TOGA application is the key. Boeing/CFM have obviously determined that application of TOGA with N1 below 40% can have unpredictable results, so logically 40% is the minimum. I don’t see any problem setting thrust higher than 40% and then pushing TOGA. For example if the thrust was set symmetrically at 70% and then TOGA applied, the maximum possible differential thrust would be approximately 20%. ( or Takeoff N1-70). If takeoff thrust was set manually and TOGA applied then maximum possible differential thrust would be zero. If 40% was manually set and TOGA pushed, max differential thrust would be 50%.