I tried a slightly modified version of the way you described a few weeks back: rather than a single slit, I made two: the sleeve head was very narrow already, and a single slit down the middle would have resulted in a pointy church window effect! So I slit in at an angle from the 'corners' of the sleeve head - more or less where the two dots for ease stitching are - and took half the reduction from each. This worked perfectly! Just enough ease (1") to get the sleeve into the armscye without any ease stitching or puckers! Yay!
This should make tailoring this bit of silk dupion a great deal easier than the last lot! :) And, like you, I shall measure all sleeve head and armscye seams on set-in sleeves to check for that 1"-1.5" ease allowance.
THANK YOU! :) :) :)