A little stuck

This is a classic Stephaniesque moment. I’ve reached the armhole of the tank, and as I was about to begin it, I realized that I have no idea what I’m doing. None.
I thought that I would simply cast off the right number of stitches in the correct place then decrease in an appropriate curve until the armhole was deep enough and wide enough. Good plan eh? What was I thinking? The “right” number? The “correct” place? What did I think? That this mystic information would come to me in a dream? That upon reaching the armhole I would be suddenly filled with the cosmic armhole knowledge that I lack? I have sat very quietly with the tank in my hands for some time, but the answer hasn’t come to me. This can’t be how design works. I bet this never happens to Bonne Marie, I bet she was born knowing. I bet that when she gets to an armhole it all just feels right.
The options as I see them are as follows.
1. Just try it. (I don’t know what “it” is, but maybe the vague armhole voodoo will spring fully formed into my mind as I knit.) This plan also involves me reaching deeply within myself for the acceptance that this will include a lot of frogging.
2. Scrounge around the house for a pattern that has an armhole like the one that exists in the recesses of my mind and try to somehow extrapolate that into something that would work on my tank. (To make this one work I have to forget the fact that there was no tank pattern with the right thing going on or I would have knit that and avoided this whole thing.)
3. Go drive around in Ken’s zippy new car that he loaned me while he’s away. (This might not help, but it would be fun)
4. Appeal to the knitters who read this blog to impart the principles of armhole enlightenment.
5. Shove the tank into the bottom of the knitting basket and spin this. (More fleece artist. You didn’t think I just got what I showed you yesterday, did you?)