From: Washington, DC
If you want to set up a training air group, is it generally better to use the training group mission with a training focus selected, or a group mission with a certain percentage of the group on the training patrol level? What is that percentage of the group that is on training actually training for?
So to answer your original question: If you want to train (that is, develop pilots' skills), use the Training mission, 100%, range 0. There is no such thing as a "training air group," if by that you mean a group that can only train. Every air group can be assigned a training mission (and thereby become a "training squadron"), and in 1942 (and even beyond) every air group not actually fighting should be training, 100%. Train train train! Training Mission, 100%. Accept no substitutes.
If you set certain missions available to a given air group, you can hold some planes back from that mission by setting a percentage under Patrol Levels, as I held back half the planes in the Marine squadron in my example. In my experience, and meaning no disrespect to anyone, holding planes back from a mission for the purpose of training is a waste of time.