In reading any old pattern tailoring book, I came across this :

https://archive.org/details/b1108865/page/n15 (page 15)

Which mentions a pre-defined ‘square’ used to calculate the angles for a certain lines (like on the page given the ‘seat angle’ for a pair of trousers).

Not a priority, but does this have an equivalent tool in Seamly2D? Alternatively I can possibly examine his sample given prior to this in the notes and get the relevant angle by trig functions.