Aren't Eureka moments great? I have another suggestion, a simple one. Since roll film adapters shoot, at most, 12 shots, just finish off the roll! If you're half-way done with a roll and storing it, it's not getting any better sitting in your bag. I always used to worry about saving that last foot of film on the roll and then I realized that the film was often (for me, not you) so poorly stored in my bag that it was deteriorating. I usually can find a still life to shoot a few images to finish a roll, no matter what the subject of the first half was- or I use it to test a new lens. I know everybody has their own work flow, and the keeping and value of film is a personal thing, but I feel I became a lot more flexible when I stopped leaving half-finished 120 rolls in my camera bags. I can always pop in a new roll and start a project with the right film.