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Roger Moss

Just for the record
I use NGK Iridium BR8EIX for racing with 21 deg BTDC ignition
I would use BR7EIX for a tuned iron engine. ie gas flowed and ignition back to about 27 BTDC
I would use BR6EIX for a standard engine with lower compession pressures and thus lower burning speed and ultimate temperatures. Ignition advance standard ie 34 / 35 BTDC.
Note that if you improve the breathing you must retard the ignition or you will risk detonation, overheating and seizure.
If you want to check timing set TDC, make mark on flywheel, use flexible thin steel rule bent over flywheel, mark every 2mm. This is almost exactly 1 degree on a standard flywheel, so you only need to mark as far as you need.
Obviously you choose a reference line for zero on your crankcase and lay a rule or strip accross, or make a pointer.
Kind Regards