You set the tension by loosening the generator and pulling (prying) it away from the block to tighten the belt. 1/2" deflection is good. Then tighten generator.
Tighter is not better. 1/2 inch deflection is very tight, less is to tight. At one full inch, less stress on water pump and generator bearings, but it wont slip.
Don, no argument, don't want it too tight. Ford 1955 Shop Manual sez "For V-8's half inch inward deflection with normal thumb pressure". Whatever "normal thumb pressure" might be...