Primary Chain Adjustment

This is so simple that I am embarrassed to bring it up. But at my age memory of details is beginning to fade . The service manual states that the slack in the primary chain is 3/8” and a total of the up/down movement in the middle of the chain. However, a motorcycle mechanic who owns a Commando recently told me that
the slack is 3/8” up and 3/8” down for a total of 3/4” slack. I had always used the former. Which is right?

C41,6. "....total up and down movement of 3/8 in..." is clear enough so I believe the mechanic is wrong.