AFAIK band-on derailleurs are available in 28.6mm, 31.8mm and 34.9mm.
With this in mind, the answers to your questions are:
Digital callipers are available on eBay for a fiver or so.
@Scilly.Suffolk started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
AFAIK band-on derailleurs are available in 28.6mm, 31.8mm and 34.9mm.
With this in mind, the answers to your questions are:
Digital callipers are available on eBay for a fiver or so.