OK. I'm taking another shot at this tank bag deal. I called Twisted Throttle and had the nice lady get a bag in her hand and do some measuring. I settled on the "City" bag. With the lock ring placed as far forward as possible, it's about 3 1/2" to the bend in the bottom of the tank, and that's close to the distance from the gas cap to the back end of the tank and she said that the dropped portion of the tank bag drops at about a 30-degree angle from the horizantal, which is close to the angle of the back of the tank.
I'm keeping my fingers crossed and burning candles. If you can set aside some time for special votive offerings on my behalf, I'd appreciate it. Twisted Throttle says they're willing to give me this one last chance, but they think it's time for me to get my act together.
Seriously, they've been great. Oh, and they sent me a free Twisted Throttle t-shirt to make up for the Givi missing part fiasco.