If I connect points K3 and X1 with a line to draw the side of the sleeve, and then change the arm length (KX), the line I drew no longer reaches the end of the sleeve.
How can I ensure the side lines reach the end of the sleeve when changing the arm length?
The issue is you have connected K3 to a point D5 not X1… and K2 to D6 not D4. Not sure where D5 & D6 are… they are not in the patterm you attached.
BTW… it’s not neccessary to connect points with a line to create a pattern piece. The line tool has 2 functions… 1) Provides a visual connection between two unrelated points. 2) Creates length & angle variables between two unrelated points.
“Lines” are not used to create a path for patterm pieces… only points and curves are.