Hi. I don't know where it's written in your manual, but this is extracted from a PAPI installation manual.
Commissioning of PAPI
approach slope indicator system
10.6 – Aircraft threshold crossing height. The aircraft threshold crossing height is the difference between the eye height over threshold and the eye-to-wheel height of the particular aircraft in the approach attitude. Eye path towheel path height for particular aircraft should be obtained from the airline or the aircraft manufacturer concerned Some common nominal aircraft eye path to wheel path heights are set out below:
Eye path to wheel path height (ft) Aircraft type
12 F 50, BAE 146
19 F 100, B737-200
21 B737-400, B707, DC9
23 B727
24 A320, MD-87
26 MD-80
30 A310-300, B767
33 A300-600
38 DC 10
45 B747
Source: ICAO Aerodrome Design Manual Part 4, Appendix 6)