But John, without knowing the force being applied there's no way to use that technique?I’ve been using the 3/8 in estimate or the little less than 90 degree rotation on the longest span technique. It has served adequately.
Then I came across this more specific description from a belt manufacturer.
The most common method for drive tensioning is “force deflection.” A predetermined force is applied to the open span of the belt drive. If the deflection exceeds 1/64 in. for every inch of span length, the drive is tensioned higher. If the deflection is too small, drive tension is excessive, and must be reduced.