I wouldnave thought starting with the sleeve pattern as given in your pattern would have been right if it is knit from the top down, then just stop at the length you want, with enough space for the pattern you want at the elbow.
Otherwise, could you find a similar sleeve on a free pattern perhaps and use as a guide for size?
The capped sleeve I added to my top were given in the pattern I had… and I still had to knit 6 to get a pair I was happy with.