I have been reading articles and discussions on forums that setting timing to 40+ year old manufacturers guidelines has no relationship to modern fuels now unleaded and with many additives.
I connected my Gunsons LoGauge Vacuum Gauge to the vac advance connection which was left disconnected. Started the engine and the gauge immediately read retarded. I advanced timing to max the vac reading well into the green zone then backed it off slightly. The engine tickover increased alot which i re adjusted.
When i connected the timing light it was reading 20deg advanced!! instead of the recommend 10deg. It seemed alot of advance but the car performed noticeably better on a test drive.
I was surprised at the difference and will monitor things especially in case i can hear any pinking.
I followed pretty much the procedure described here at http://www.mgb-stuff.org.uk/ignitiontext.htm#vgtuning