Great post and great question. I would like to know that also.
The best way is: measure distance A - X and place this this on the sleeve from point A and mark point X there, minus the seam ( 1/2" ).
Now measure distance C - Y and place this on the sleeve from point C and mark point Y there, minus the seam ( 1/2" ).
Now measure the distance between points X - Y on the sleeve, divide this in two and that is where point D is best situated.