Sorry Cyborg but all I can give you are approximate figures and you can choose which you think might be useful. One racing engine I have helped with has squish bands, twin plugs, Terry Prince heads and is about 1,200 cc. With all that a figure near to David's of 18 -19 BTDC seemed about right. Without the Squish bands then 21 - 24 would be about right but you have to experiment with your own riding style, fuel etc. With single plugs and no squish band, but using modern fuel, then rather than the original 39 you need something like 36 so you can see what a difference the twin plugs make. The late Phil Primmer told me that he had used twin plugs on his road going outfit on a run from the UK to the South of France and he believed that he got more miles per gallon. Problem is, I have found French fuel better in diesel cars than the UK equivalent so whether this also applies to petrol I don't know. Also Phil had other mods like a tube connecting his two carburettor manifolds.