The half-inch gain might have been your ‘newbie gains’. Although it could also be that you need to increase the intensity of your routine a little so as to keep the gains coming.
A twenty-minute set is recommended as standard really only because the hanger has to be removed to re-establish blood-flow (if it wasn’t for that necessity we’d probably all be hanging much longer sets).
Are you only hanging 10 minutes because of discomfort?
You wrote in your post that you’re not experiencing fatigue?
Why do you hang for only 10 minutes per set?
Four sets at ten minutes with a ten-minute break in between each set makes a total of an hour and ten minutes. If you hang two twenty-minute sets instead of four ten-minute sets (with a ten minute break between each) it should only take fifty minutes (which has saved you twenty minutes— almost enough for an extra twenty-minute set).
It’s generally believed that hanging in such a way as to cause fatigue is what leads to length-gains. In this sense fatigue can be considered a good ‘PI’ (physiological indicator). Maybe think about hanging in such a way as to achieve fatigue.