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If you can make it ping (advance) by turning the bean can around, you could back off (retard) from that point a little and be pretty close.

Put a degree wheel on the front of the motor. Set it to zero at top dead center. You can find tdc with the positive stop method. While at tpc mark the flywheel with a dot of paint (use a tiny artist's brush). Then using the degree wheel turn the engine back to full advance and put another dot of paint on the flywheel (use a different color). Go ride the bike till its fully warmed up then with a timing light set timing to full advance at 3500-4000 rpm (need a friend for this). Lock down the bean can.

If you try to set the timing by ear what works for me is to adjust the timing for the fastest running at around 1000 revs, then slowly retard it from there until the motor only just starts to slow down.

I haven't done this on a dyno, but it gives me best speed at the top of my test hill.

Those that set airhead timing on a dyno will tell you that there is around 6 degrees variation on what works best , from bike to bike,somewhere between 30 and 36 degrees, so the timing mark is really only a guide to get you running anyway if you are after top performance.