Equal and Opposite:

The glove arm is a mirror image of what the throwing arm is doing.
Pictured below is an 8 year old, 11 year old and 13 year old just before foot strike - for the most part, these
pitchers are getting into a good equal and opposite throwing position (looking at the angle created by the
throwing arm in comparison to the front elbow angle).

The key is that you want the same arm angle on each side. If
the pitcher has his throwing arm up at a classic 90 degrees at
foot strike, we want the front arm to also be at 90 degrees - the
front arm can be bent at 90 up, down or sideways to achieve
this equal and opposite position.

Photo to the Right: Note the 90 degree bend in throwing arm
and the 90 degree bend in the glove arm - this is the "Equal
and Opposite" balance theory.

The entire philosophy behind this is to create a balanced position to throw from (again - Proper Posture
at foot strike).
I would not get too carried away with this unless you had someone that was completely dropping the
front arm to their side as they get to foot strike (this will frequently happen in younger pitchers).

Pitchers will have any number of different arm angles, most all will be ok (we are not creating clones).
You would like the throwing elbow to be at shoulder level at foot strike.

Match the glove arm to the throwing arm.