From Vern Little on the Van's Air Force website:
Belt deflection is an unreliable method of tightening the belt.
Instead, use a torque wrench on the pulley nut and adjust the tension so that you get about 12 ft-lbs before the belt slips.
A loose belt can lead to under-voltage or over-voltage from the alternator. Overvoltage is caused by the regulator trying to force more output from the alternator while the belt is slipping. When the belt grabs, it puts a surge of voltage on the bus.
Please don't ask me how I found out!
Lycoming SI 1129A Accessory Drive Belt Tension
New 3/8" 11-13 ft/lbs 132-156 in/lbs
Used 3/8" 7-9 ft/lbs 84-108 in/lbs
New 1/2" 13-15 ft/lbs 156-180 in/lbs
Used 1/2" 9-11 ft/lbs 108-132in/lbs
Vern Little 9A