I'm gonna be using a board with a 25.4cm waist and I have a 12 boot too, I haven't tried it yet but I've gotten feedback from a guy who also has a size 12 that uses a board that's even less than 25.4. I believe he said 25.2cm, and he doesn't have any problems.
East Coast Park Rat
2008 Burton Twin 157 w/ Rome 390 Boss
2005 Burton Canyon 168 w/ Ride LS
"its ppl like you, that I would "impact damage" your face if i ever saw you
keep your mouth shut. one more time, please and thank you." -user: chingy_