Using a builder`s level, the grade for a footing is being set. The bottom of the footing will be at elevation 94.00`. A backsight to the bench mark elevation 100.00` has a rod reading of 4.20 (F128) (130) (F255). What should the rod reading be for the bottom of the footing if the grade is correct?

Using a builder`s level, the grade for a footing is being set. The bottom of the footing will be at elevation 94.00`. A backsight to the bench mark elevation 100.00` has a rod reading of 4.20 (F128) (130) (F255). What should the rod reading be for the bottom of the footing if the grade is correct?