I kept the HEM CIRCUMFERENCE at 8" to get the maximum cap width and elbow width. Drafting a wider hem will make the cap more narrow.
I used ZERO CAP HEIGHT, hoping to eliminate folds in the back sleeve cap.
To get a bigger BACK CAP AREA, I chose ARMSCYE UNSEWN as the back dart position, but DID NOT use that on the bodice.
This is how the sleeve pattern looked:
I cut the arc on that new point and moved the lateral portion to the front of the Oversleeve. I also extended the HEM LINES on both pieces to get my desired hem circumference.
Actually, my instructions are on each picture, so perhaps a picture really is worth a thousand words?