While calling for original specs is a good idea (I did this with my Boler), I don't think it is necessary in order to determine the start angle. The inner bar of the Torflex and similar axles is 45 degrees rotated from the outer square tube at zero load, so if you look at the end of the bar where is goes through the arm, you can see that its corners point straight up, down, left, and right. The square hole is the arm is made at the desired start angle.
The following image is "borrowed" from a Dexter document, and I have added (in blue) where I would read the start angle
, regardless of current axle position
, as the angle between the arm and the diagonal (line between opposite corners) of the inner bar.
This won't be really precise, because the outline of the inner bar is somewhat obscured by the welded joint, but with only a handful of start angles available, it should be reasonably easy to determine which one it is. The example has a 22.5 degree down start angle, just guessing by eye (it looks like half of 45 degrees).
If the rubber has really sagged, the diagonal line of the inner bar will be as much as 30 degrees off of horizontal with the trailer weight
, but the angle between that diagonal and the arm is always the start angle.
So, people with rubber torsion axles and mechanical inclination, does this make sense to you?