You're absolutely right on this. It's amazing how little tension is required. I think the alternator would be fine with 3/4" of deflection. After installing the new belts on the SD, after one 50 mile trip, I noticed the idiot light starting to glow a bit with the lights on at night. Found the belts had stretched so that the deflection was over 1". Still charged fine most of the time.

Most folks overtighten those belts.........to the detriment of the alternator and water pump.