The basic principal is that the weight of water in the vertical portion will not be carried by the spring but supported by the pump impeller/casing.
I don't know how to call it..."comon sense " or
" common practice " because it's governed by the laws of physics.
For smaller or even medium size pipes it's not a big botheration because the difference in spring settings is not much. But for cases such as 36" pipe the difference we are talking about is 15KN which is significant.
Also by not taking the weight of water into consideration we are safeguarding our WNC case wherin just before the start up the line is empty but the spring locks are removed.For big bore piping it could lead to large upward thrust effecting the pump alignment. Could be I guess...!