Hi Jeff,
Interestingly enough, just to contradict Reg's 1930 book of the Douglas, I have checked with the 1929 S6 Douglas Motors, Care and Maintenance Manual where they claim that the correct setting for both inlet and exhaust should be 0.006".
If you do not succeed in finding your manuals, the Douglas Club does have a stock of copies of both the manual and illustrated spares list. Unfortunatly they are printed from early photocopies but are still perfectly legible. Cost is a mere £7-00 each plus the post. If you decide you would like one or both please email direct to my email shown.
Dave