Outer Limits 4x4 Board

just wondering where abouts the mixture screw on an impco 300a gas mixer should be. towards the rich sie of half or the lean side of half. the gas mechanic where i am puts it on half and the motor backfires and runs like shit. I lean it out and it runs great.
any info would b great

mine is slightly towards the right side, its halfway between the middle marker and the next one towards rich. if its any leaner, it doesnt seem to rev as good, and any more richer, doesnt make any difference in my opinion so i just leave it there and runs good