The siting of PAPI wing bars is arrived at by calculation. Airport Lighting Specialists are able to calculate the correct distance from the runway threshold to the PAPI wing bar . A simple method to establish the approximate distance is by use of the PAPI estimator that can be downloaded from this page.
The performance and calibration of Research Engineers PAPI projectors, clinometer and veracity of the relationship between the two can be verified on the ground, cheaply, quickly and independently by a licenced surveyor. The method used is to observe the PAPI transition, note the mid point, construct a parallel with it, and to measure this directly with a theodolite.