Hello Keith, at 4mm/ft, 12 mil is 3 feet, so prototypically they’re the nearest. 10 mil are, at 2’6″ ( according to Paul Bartlett’s photo ) seriously undersized – it might be to allow greater clearance on curves? I can’t advise which one to go for, I’m afraid š – but 3′ ones are far more common, so you would stand a better chance of being able to use them on other models, as opposed to 2’6″ ones, I’d think?