Intro is under review.
Users of the program are young sailors, who are sailing in a dinghy (the 420), and who are measuring things on their boats, on shore, when they set up the boat for the conditions of the day. They measure several length values (including Mastfall) and the forces in the wires (Wantenspannung). But this is tedious, and accuracy may be poor because of friction in the system (hysteresis) and because they are usually taking the length value not at the most appropriate location. For example, length is taken from top of mast - using F0F instead of F0C. At point F, there is bigger elasticity in the mix, and it depends on the bending of the mast, when it should be rather independent of any other target value.
There may be a better approach to the task - the friction free simulation, which users could do at home. They will measure less often, while getting more consistent, reliable results.
The program is an abstraction, an approximation. It uses a simplified model and the math behind it is basic, at least in the most basic model variation. To compute how the mast will bend is intermediate in complexity, I would say. The current implementation of the program is pretty good at that.
My old desktop application from 1996 used a user-provided curve (line or spline) to look up the force. You only needed to specify the params for the curve (Weg-Kraft-Kennlinie), based on what was measured on the real boat. The Trimm420 app in the store does not currently deal with the forces, it just does the kinematics of the model.
Users of the program do not sail faster, simply because they use the program, but they will get a better understanding of how things work in the rig.
Confidence based on knowledge will allow them to perform better in the race.
( Once the boat is out on the water, all sorts of things including wind and the waves will deform the rig in a way the program cannot predict, this is outside of the scope of the program of course. )
Users can compare measured values to the prediction of the program, and change the model data so that the prediction matches real values closely. The program may also help transfer settings (trims) from one boat to another.
There is a Facebook group: Trimm420, maintained by the Federgraph Facebook page. But note that this group is new, no users there, not at the time of writing, in August 2019.
Of course, if you just wanted to know how long the new shrouds should be for a similarly rigged boat you could use the App and work with updated data in the Trimm-File.