I'd like to adjust the timing on my 65 dart. 225 /6. (1977?)

The original repair manual says to set to 2.5. I think it was BTD but it could have been ATD. It's in the book I'll relook it up before adjusting it. However, the tab with the marks on it has 10 and 5 painted on on the other side of TDC from the manuals setting from a previous owner. and when reading a recent post at /6 forum. They were swearing by 16! which isn't even on the tab.

http://www.slantsix.org/forum/viewtopic.php?t=27635&postdays=0&postorder=asc&start=75

"The advanced timing comes in handy to complement the fuel correction. I have tried to run around 10 degrees but I get better economy at 16 degrees. It has a little crisper response to the throttle changes. The gentlemen that owns our local carb shop swears that 16 degrees initial is the ticket for a stock or nearly stock slant in terms of best mileage."

So how do I know what to set it at? I'd like to go for MPG versus power.