Turning a L.H.external thread without an undercut

I need to cut a L.H. external thread on a set of brake rods - all the videos on youtube are cheating by using an undercut to start the thread - the material is 10mm. dia. and I wish to cut a 10 x1.5 thread about 50 lg. - Any advice would be appreciated.- when I cut a thread I usually leave the half nut engaged and reverse the headstock rotation - I offset the compound rest but in this instance the CR would be angled towards the headstock but do I set it at 29 or 31 degrees?
Alf Scotting (Taree NSW)