I’m aware of that, but in the diagram in the book, that’s not technically what the ‘square’ is doing in this instance. It’s being placed to get what it terms the ‘seat-angle’, for a line that is NOT perpendicular to the line measured as the “seat” in the pattern ( I am perhaps wrongly thinking this translates as the hip/highip line in modern terms). I can find the angle with some trig functions I think.
Thanks for the suggestion BTW
An Update, following the measures given in Vincent’s drafting (https://archive.org/details/b1108865/page/n12) , I’ve determined that what the square is providing is most likely an angle of somewhere between 68 and 72 degrees for the seat angle… (most likely 70 degrees, but I couldn’t confirm this, with the precision of measures given.). Currently Seamly2D doesn’t have a known “measurement” for this, but I could set a custom one.