Rest your hand on the side of your hip, with elbow bent.
I started a sleeve back in October.
The length of the sleeve and its relationship to the piston skirt dictates the maximum, durable stroke possible.
The path of the tape measure should follow the outside of the arm.
And when I extended one of my arms and the sleeve is ending at the wrist as well.
You can tweak them further according to the design you wanna wear.