I am having a hard time understanding how the seat angle is constructed in the pdf of the pattern. Point 23 is created by drawing a line through point 2 and some unnamed point, which I don't understand how to construct. It also seems like the center back line ends at a fixed height. Would the endpoint of the center back line not vary between somebody with a flat/larger behind?
Any help is appreciated.
Both the centre back and front will end up opposite from hem to knee. But I can't guarantee belong that. You will need to lay the crease lines on the stripe too.