I hopped downstairs and pulled on the body of the sweater-so-far and grabbed a Sharpie out of the pen-jar. Wonderful things, Sharpies. At this point my daughter started to pipe in with the obvious questions, "What are you doing with the scissors, Mommy? Why do you have the pen? Why are you drawing on your shirt?!!" (At which point I took a moment to explain that you should never, EVER, ever do this to your own shirts, and it is only for Mommies to do and .. hmm.. this probably should have waited until nap-time, but too late now! -- If I ever find a size 4T shirt sacrificed to the "sweater gods" I'll only have myself to blame.) I traced what I could of the armhole of the sweater right onto the shirt.
Replacing said beat-up-old-drawn-on-shirt with something slightly less beat up (hey, I was having one of those days where only the oldest, softest and most beat up comfy shirts would do) I flopped the poor thing down on the floor and applied the scissors to the shirt (remember, sweetie, you never EVER ever cut up your own clothes and shouldn't touch the scissors without Mommy around..). I snipped up the inner seam and laid the whole thing out flat. There! I had my correct arm-cap shaping. I then measured the thing, and figured out how to knit it. Viola!
It even worked. I finished knitting it this morning and carefully seamed it onto the body from the top of the shoulder down to the armpit. I didn't bother with the long seam under the arm, because this was purely a "I really hope this worked!" moment. It may not be the most amazing design ever, but it did work! I'm not sure if I don't want to take it off and clean up the lines a little bit. I know for certain, now, that I will be popping open the side seam and either adding a cable-strip all the way down to the hip, or at least a gusset under the arm because it is way too small around the arm.
But at least I know it worked! We're on to the second arm to see how much yarn will be left at the end, and if I need to risk picking up one extra skein at the LYS to finish things off.