Im running 21* of timing!

[withamc]

Even if you can advance it further, there's a point where it actually starts costing you power. You want to advance it to the point where it's igniting the mixture in the combustion chamber just prior to the piston beginning it's downward stroke. If you keep adavancing the timing past this point the combustion actually begins to slow the piston on the compression stroke. I can advance mine past 16* without pinging, but 16* is where it performs the best.
3/8" of slack in the chain seems like a lot. I don't remember having much slack at all when I put my cam in a few months ago.