Advice I received from a very reputable engine builder is don't go above 7.5:1 for normal road use and current rather dismal petrol. With any engine I build now, I go for the lowest available compression ratio pistons. Modern petrol has a considerably lower octane ratio than was available when our bikes were new. You may be OK with a higher compression ratio if you always use premium petrol. However it isn't always available. You try getting some North or West of Perth for instance. I take it everyone uses premium petrol in their BSAs. If not, why not?