That is why, as a first choice, I would size the spring to put zero dead weight on the flange in the installed condition. It allows an easy check by the installer. If the flange bolts are loosened and the pipe floats rather than being launched skyward, then spring carries the real load. If not, the installer should ask if he can reset the spring (under supervision) until the pipe does float on the nozzle. This might not be the lowest operating load that can be achieved in theory but it is the only one that can be easily guarateed to be 100% accurate and to not affect shaft deflection. (A load other than zero installed load can be achieved by adjusting for zero installed nozzle load and THEN adding / subtracting the required difference).

When the flange bolts are retightened the pump casing should not be carrying any unplanned load and shaft algnment should be as near that of the bare pump as is practicable.

For very heavy loads and those using constant effor springs, the only practical method is hire a load cell, weigh the pipe at the hanger position as-installed and THEN factory set the spring. Pipe stress calcs are probably subject to more tolerance than the pipe weight.

Does somebody have a comparison between installed pipe support loads and calculated pipe support loads in a real pipe system ?

The alternative is for the installer to tighten the hanger rod until the spring indicates the theoretical reading. Bending something or failing to achieve shaft alignment or both is virtually guaranteed.