The service manual setting at the "BB" or "ball" is a ballpark guesstimate to get the engine running. If you read further, you'll see that the "octane selector" slot was factory-designed to tweak the timing for variations in fuel quality, altitude, different loads on the engine, etc. Most trucks were designed with fairly low compression and not much timing advance so they could run low-octane (translation "cheap") gasoline, and run full throttle for long periods of time against a maximum gross weight load without detonating. Now that the vast majority of our trucks are Cowboy Cadillac toys that seldom if ever see a heavier load than a right-seat passenger, and we have better quality gasoline than they did in the 1950's, we have a lot more leeway where ignition timing is concerned.
Jerry
