Standard sleeve cap construction for set-in sleeves creates a bell shaped curve that fits in to the top of the sleeve. I like to use a set-in sleeve cap that picks up stitches around the armhole that then uses short rows back and forth to curve the top of the sleeve to create a sleeve cap.
It's much easier to create a good fitting and tidy sleeve cap using this method.
I've done a video showing the basic idea behind this type of sleeve cap:
Why not give this technique a try in one of the following patterns:
Read more about Short Row Knitting.