Hello Adrian – you should refer to an article by Mark Ordish entitled “What’s Wrong with 2 Speeders?” in Yowl Vol 23 No.9 (April 2004) page 305. This was of great use to me when restoring my 1929 sports.
Mark suggests a coachline of no more than 2 millimetres and this is what I have applied to my bikes since. I find that white is a little too much and an ivory or off-white less stark.
The Yowl concerned is available in the archive in the members area of this site and includes pictures of two seemingly original 2 speeders. How are you getteing on with the rebuild ?