A small set covers most needs: if/then for logic, score for turning choices into points, floor and ceiling for rounding, and distance between and altitude of for location maths. They turn a form that merely records answers into one that flags, scores and measures as it goes.
Video: Master mWater Formulas Like a Pro on YouTube
Calculations left for the office are calculations done late, and sometimes not at all. A form that scores a facility while the enumerator is still standing in it can raise a follow-up before they leave, and one that computes days since the last visit can decide on the spot whether this visit counts. These formulas are the difference between collecting data and acting on it.