Even wrapping masking tape around should give you quite a good guide to follow by eye.
This. It's the only way I can cut or file in a straight line!
Only other thing would be if it's a 28.6mm seattube and there's room, stick an old 1 1/8" stem around it and file down to that.
@M_V 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.
Even wrapping masking tape around should give you quite a good guide to follow by eye.