Use one finger, with moderate pressure, (you don't have to dislocate your finger at the joint), a belt should deflect about 1/2" for pulleys around a foot apart. If they are real close together (a couple inches between them) the deflections should be about 1/4"
I use one of the Kent Moore belt tension gauges, according to an OMC service bulletin I read, they changed their spec from 1/2 to 1/4 in deflection, or 75+ or - 10 lbs on the belt tension guage. When adjusted to 75 lbs, it's just about 1/4 in deflection.
