As I was working on my 1780s blue silk jacket (inspired by a 1790s jacket that I own, naturally) today, I noticed something--as in all my other bodices, the sleeve cap didn't match the armscye. This didn't bother me as it's what I always do. I had tried the jacket on, and was happy with the sleeve fit, so knew the mismatching wasn't an issue.
Every fabric behaves differently. Even the same fabric behaves tends to behave differently when made into two different sleeves--it's rare that I set two sleeves exactly the same. They usually want to do something a little different! I firmly believe in close enough when it comes to patterning sleeves. Get a sleeve that pretty much fits, and fix it in the final fabric.
Now, I wish I had a slightly less messy fabric than this brocaded taffeta to demonstrate this!
You can see the shoulder strap and the sleeve cap is extending past it on the right. Initially, as I usually do, I matched the edge of the sleeve cap to the edge of the armscye edge. Then I tried it on (I recommend parallel pinning to do this!) and looked at where the sleeve bulged, and then pulled the extra fabric in.
In this picture, I'm folding the sleeve strap into its normal place.
And here's the sleeve strap folded into place. I will trim the seam allowances to match, especially since in this jacket they're going to be pressed towards the bodice and the lining will cover the raw edge!
And just another angle of the same. The strap is on top, with the top of the sleeve extending below.
I hope this is a helpful technique--I know when I started out and thought that I had to match the cap to the armscye perfectly, I had quite a bit of difficulty with sleeves. Now that I don't try for that, sleeves are no more daunting than anything else!