Which mark on the harmonic balancer do you set the timing for on a 4.3 V6 Chevy motor

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I set the distributor at#1 plug at 6 oclock position like the book indicated, #1 plug sparks when manually turning distributor with rotor pointing at 6 o clock position and the cap with the number one plug wire starting off in the number 6 position and following counterclockwise from 1 to 6 in that order also I have a discrepancy whether or not top dead center is the first or second mark on the harmonic balancer apparently the Piston comes up at top dead center with the mark on the right but the book says to set the distributor to the mark on the left not sure how this affects top dead center