Raphael--I have used and surveyed all foundation types. If I can measure a deviation from level, I correct the post length. I use a 1947 Brunson surveyors dumpy level. Error is minimal with proper adjustment. The obvious adjustment is the level vial, the less obvious but profoundly important is to adjust the crosshair to the precise optical center. Without proper crosshair adjustment, the level will sight a conical surface not a level plane. Learn to verify optical and laser instruments with the two post method. With proper crosshair adjustment, the instrument will sight a plane, how level is the plane is the function of level vial precision and adjustment. Builder levels and transits should sight a plane at + or - 3/16" in 150', better precision is not needed. So chart the deviation from the sighted plane.
bman--I have used the Benson method and have mostly moved on to drawbore.
