I've retracked many frames in my lifetime. The bicycles that I usually work on are older machines where the seat tube and down tube are the same diameter. Therefore it is easy to take an accurate line across the seat tube and down tube.

However, on the MTB that I currently have in my workshop the seat tube is narrower than the down tube which makes it impossible to take an accurate line using my usual method.

Can anybody offer me any advice on how I might be able to retrack this frame accurately?