I have been testing head clearance for the last few weeks on BRB and Bi metal. I have came to conclusion that in HK, 30C & 80% Humidity, the best is around 0.65 to 0.7mm head clearance which produced the best overall power for 30% .
I actually have problem with 0.5mm head clearance. The needle was too sensitive and no top end. I was using Odonnel 97T plug and can't tell from reading the wire (may be Odonnel plug too good )