How much timing help

[jrr]

The builder of my new carb wants me to dial in more initial timing, going from 12-13 to 18-20 degrees. He feels that with my cam, I am "firing and dragging in about 10% exhaust gas". I am getting a below 3200 rmp lack of sharp response and gas mileage is around 6 mpg from normal 12. Plugs are black on edges and a little damp.

The set up is a little 351/385hp FMS crate. GT 40X 305s, Vic jr, Z303 cam 228/228. Vacuum advance set up with total at 3K of 34 deg. Basically street use at this time.

What do you folks think about the increase in initial suggestion???

Thanks