After copious amounts of head-thudding, I finally managed to persuade a spreadsheet to show what I want. It's too big to fit on one page, and refuses to print in landscape mode, but at least the thumb is in the right place and it's roughly mitten-shaped with the pointy bit at the top where it belongs.
Now to attempt knitting from it. If that works, I'll share.
If not, it's back to the head pounding. At least I've gotten good at that.