Justin,

In an attempt to split some hairs, my reference shows 6&deg BTDC UNLESS you have a AIR pump, then is shows 4&deg BTDC. It's OK to customize these figures for maximum 'poop' without negative side effects...overheating or pinging.

The backyard credo is 'more is better', within reason.

I don't think I know of any engine that is timed to fire at TDC.

Stuart