When are people taught about a set point and trumpet playing?

ALL beginning trumpet students ARE taught the concept.
As first year trumpet students the range falls between low g and 4th space e. They are taught to set for a second line g. This prevents the need for lots of shifting, curling......
The problem comes in year 2 when the range of the trumpet student increases the center of range starts moving up. The set point should as well.
Non trumpet playing teachers are the reason this and a hundred other things are not taught. If you use a pivot and lower your tongue as you descend the g on the staff setting should be easy to add to your full time trumpet playing.
The concept is about setting closer to the needed top note than the needed bottom notes. In some groups setting on a middle c might be enough. In another ie playing Si Si MF (up to g over double c) setting on high c might be better. There is an old Big Band chart Harlem Nocture where it is almost all below low c. The set point on this is of course lower (second line g).
If you can set for about an octave under the top note played then you get the benefits. This is a flexible IDEA it moves according to the range needed in a piece.

Less lip movement means less work. We all know that the less lip movement we use the easier playing is. I don't mean corner movement but the constant tensing / releasing compressing of the center.
I have always taught it this way in the past. I like to use page 125 line one of Arban. It is a c major scale up and down. But every other note is low G. It both starts and ends on the low g.
1. Play the low g then do the exercise. Most people have trouble on the top of the scale.
2. Play middle c then play the exercise. Here most people have trouble with the low g's.
3. Play a second line g and then play the exercise. Most people can cover the range spread very cleanly this way.
This shows the idea of a lip set in the middle of the range of the piece. If we set on the low end then we have to waste a great deal of strength to play the higher end. (Here it is only middle c.) If we set for the top note the tone and response of the low notes may suffer. (In younger players)
By setting in the middle we compress the range and don't work the chops.
This applies to ALL playing. In a piece with high c's in it set the chops for a note closer to the top like the g on top of the staff. That would compress the range of the same exercise played an octave up.
Compressing the range makes a piece more playable.
I use a 3 octave g scale from low g to g over high c. If I set for low g I can't play it. If I set for g on top of the staff and use a good pivot to make the low g clear I can play 3 octave scales.
To teach a more constant lip setting. I use Etude I in Clarke Technical Studies. A low note NEEDS a big aperture but a loud note CAUSES a big aperture.
Take a second line g and play it pp. Keep the aperture the same but add a lot of air and you CAN get a loud g on the staff.
I try to teach high note apertures by having the student play soft midrange notes then increase the air. Some have trouble controlling the aperture. So I wanted an exercise to teach that control.
Clarke I uses decrescendos when the notes descend and crescendos when it ascends. It changes that only when changing register (lip set point) like going to the high c part. I've been able to use this to better teach using air as an octave key.

This is described in several books found on my site.