Timing advance help

1978 Mercruiser, 165hp L6. I've set my initial timing to 6 BTDC. I've never checked my advanced timing so I thought with my new timing light I'd check to see where it is. I just want to make sure I do this right. The advance curve in my manual (attached pic) indicates "Distributor advance curve charts do not include the initial engine timing. Basic initial timing must be added to chart for total advance curve." So the way I read the chart is I will rev to 2800 rpms and my advance should fall between 24° and 29°. Adding my initial 6° my target should be 30° to 35° total advance. I've run across other threads stating my total advance should be around 22°. Big difference. . . so am I reading my manual correctly? I know with cars 30 to 35° is normal. I don't want to screw up my engine. Any guidance would be appreciated. I've had this boat for years and have never set the distributor by the total advance numbers and it seems to run fine at cruising speed. Will I see a big difference if I do adjust it to reach the specified total advance? Thanks

Thanks for the reply. This curve is from Mercruiser manual and I assume is for the stock distributor. One thing I completely forgot to add, I changed my distributor a while back to a Mallory marine distributor (I can dig up model number if needed) so I'm guessing this curve wouldn't apply any more? Or would it? If not, how would I know what advance to shoot for?