Thanks for all the advice everyone.
I hate to say this, but I'm not terribly worried about wasting a frame or two of film on every roll. It's more about the convenience for me. If it means I have to buy a new 100ft roll a week earlier than I normally would then I'm perfectly okay with that. I'll just keep a few rolls of factory-canned film for emergencies.
I did a little test with a roll of exposed film:
- Put pre-exposed dummy film into bulk loader
- Wind 13 cranks onto roll, mark in sharpie which parts of film (would have) have been "exposed" by loading )leader, trailer, etc)
- Load film into camera, with sharpie mark best case scenario how much of the leader has been exposed.
- Put camera into Bulb mode, then start taking photos.
- For each photo, turn the camera over and mark and number the exposed area with a sharpie
- When you're done, roll the film back, remove it from the camera and lay it out.
Obviously don't do this if your shutter is going to be damaged by swinging a sharpie around the inside of the light box.
You should have a pretty good idea of the best case scenario of what is exposed when. With 13 turns I was able to get 10 frames with a minimal leader. That might be cutting it a little close, so I think I am going to live with 14 or 15 turns just to give myself a little leeway to not damage anything. At that point I can safely get 10 shots out of the camera and maybe an 11th if I'm lucky, but I would know not to depend on that 11th shot.