@Carl Love Thank you!

We thought of a solution any home sewist can duplicate. The idea is to model your ideal cone sleeve position in 3 space and measure the lines from the centerpoint of the cuff ring in 3D to the points along the new armhole. We're going to try it later.

We'll use the the pythagorean theorem to calculate this x, y, z point in 3 space:

L = sqrt (x² + y² + z²)

where L is the length of the sleeve cone.

Here's the steps we envision:

1. Adjust the armhole pattern as needed and construct a muslin bodice with the new armhole. Place it on the dress form, making sure that the top shoulder seam side seam are correctly aligned.
2. Roll up some poster paper into a cone shape and insert the arm into the cone. Make sure the cone edge makes contact with the body at every point, but does not bend. Adjust the paper cone as needed to make this happen.

3. Draw a ring on the paper cone approximately where the cuff would be, and mark the side center and back center points of the cuff ring.

4. Measure the length of the sleeve cone. This is L.

5. Move the arm forward and back to guesstimate the approximate arc of arm swing and settle the arm, not perfectly at rest, but a little forward and outward. Kathleen Fasanella, a professional pattern drafter, implies this is the anatomically correct position in her book The Entrepreneur's Guide to Sewn Produc Manufacturing, although she says human arms hang forward and down, not a little outward. I think there's also a slight outward rotation. Readjust the arm cone as needed so it's still making contact with the body along the edge, then tape the paper sleeve cone closed.

6. Measure the distance from the side plane of the body to the center side of the cuff ring in the same plane, and to get the outward rotation, measure the distance from the side plane of the body out to the center back of the cuff ring. The first distance is the forward rotation and the second distance is the outward rotation.

7. Solve y = sqrt( L² - x² - z²) to get the correct y coordinate. Now we have the ideal position of the center of the sleeve cuff in space.

8. Place a candlestick with wax candle at the center of a surface whose center is y inches vertically down and x inches horizontally out (chair plus phone books?) from the center point of the armhole. Measure z inches up from the base of the candlestick and mark the wax candle at that distance. Using a knife, slice off the top of the candle at the mark then place it back in the holder in the same x,y position with respect to the armhole center point.

9. Measure from the wick of the candle to several points at fixed distances, say 1/2 inch (or maybe less), along the armhole edge to get the different lengths and write these down, in order as you move along the armhole edge from front to back side seam. Make sure to also get (and note) the shoulder seam distance.

10. Using a 2D draft sleeve pattern piece with extra room at the top, measure from the center point of the cuff up to sleeve cap and mark each of the distances from #9 along the same fixed 1/2 inch (or less) distances along the edge to get the ideal sleeve cap shape. This shape has no ease so you to add that in if needed but you probably won't need much.

This is a lot of work. I really wish a computer could model it. I hope the above math is correct. We'll try it out and debug it.