Thanks for the suggestions guys. Much appreciated!

I did the rollout measurement, which came out at 2050mm, but was still giving an incorrect distance. Over the last few days I've been fine-tuning the value over the same measured distance, and now have it set at 2010 which is almost spot on — and close enough not to worry about any minor difference.

This setting is actually closer to a 26 x 1 tyre, so I dunno how Sigma programmed their defaults.

Anyway, problem solved.