Buy or borrow a piston stop. The cheapest I've found is the $7.88 unit, part # PRO-66792 at www.summitracing.com .

Remove all the spark plugs to make the motor easier to turn over by hand. Screw the spark plug adapter into #1 cylinder. Turn the motor over in the direction of rotation (clockwise as you face the front of the motor) until you have the notch on the damper at about 12 noon. Screw the inner rod down into the adapter until it bottoms against the top of the piston. Make a mark on the damper where the timing tab is pointing. Turn the crank counter-clockwise until the piston comes up against the inner rod again. Make another mark on the damper where the timing tab is pointing. Halfway between these marks on the damper is TDC. Use your 6" dial caliper to locate it exactly, then mark the damper with a centerpunch and put a dab of orange paint in the centerpunched cavity.

If the damper is 6.75" diameter, each crank degree will be 0.058902", so for instance if you wanted to mark the damper for 6 degrees and 12 degrees timing advance, you'd make punch marks at 0.353" and 0.707" TO THE RIGHT of your TDC mark.

If the damper is an 8" diameter unit, each crank degree will be 0.069811", so your 6 and 12 degree marks would be at 0.419" and 0.838" TO THE RIGHT of your TDC mark.

Use a dot of white paint in each of the advance punch marks and you're good to go.