NelisB I doubt that the square coils have anything to do with the pressure profile at the piston.
You may doubt it, but it’s physics. The force exerted by a spring depends on its characteristics, including section (and the shape of the section).
The pre load is determined by the length of the shaft as the nut is tightened to the end of the threat.
Sorry - cross edited… but yes, the pre-load will depend on how tightly the spring is compressed when the lever is not pushing on it (and on the characteristics of the spring); if it is “just” wound to the end of the thread, then it’s easy to replicate; if the thread is “longer” than it needs to be to allow for tuning (e.g. situation common in many motorbike shock absorbers), then it may need some measurement (or ideally knowing at what torque the nut is tightened, then use a torque wrench).
