The compass that Seamly works with is a full circle that starts at 0 on the right and rotates all the way around to 360 which is the same place as the 0 on the right. 90 degrees is straight up, 180 is to the left and 270 is straight down. These are all from the centre of the circle which is the node (e.g. A20). So the angle of the line will be measured from the centre in the direction that the line is going in and shown if one would place a compass over the drawing.
If you look at the picture below, point A21 is above and to the left of A20, the line passes through the 123 degree mark on the compass:
So that’s basically it. That’s why you need to have the reference lines for the rotation - so that you can deduct the one line angle from the other line angle to rotate the pattern by a certain amount. (It’s just the way I do it - I’m sure there are more scientific methods, but this is easy for me 
)
I hope this will explain it very nicely to your hubby
There are also all sorts of functions one can use in the formulas if you are really mathematical:
