Can I make a suggestion?
First, cut a sample of your velvet 10" (25cm) long. Gather it with the method you're going to use and pull it up to the finished puffiness you desire. Let's say it makes it 7" long. So that would means for every finished distance of 7", you need 10" of fabric. Ratio is 7:10. The inverse of that is 1.43 (10 divided by 7 or non-gathered divided by gathered)
Now, measure the front and back pattern along the arm scye seam line between the notches. Don't forget to exclude the shoulder seam allowances. Get an exact number. Let's say it's 20cm (or 8" for you non metric folk ;))
Multiply that combined length by your inverse - 8" x 1.43 = 11.44"
Now, measure the original sleeve pattern along the seam line between the notches. Let's give it a measurement of 35cm (14"). Subtract 11.44 from 14" and you're left with 2.56" (2 1/2" is close enough). That's the amount you need to remove from the sleeve between the notches to get the right amount of gather in your modified sleeve cap.
Clear as mud? Ain't math grand :)
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