If you really, really want an accurate chamber volume, get a piece of 1/8 or 1/4" Plexiglass or polycarbonate(Lexan) sheet big enough to cover the chamber. Drill a 1/16 or 1/8" hole in a spot near the edge of the chamber. Spread a thin layer of grease or petroleum jelly around the chamber edge to seal the plastic, position the fill hole right at the edge of the chamber, and start filling it with one of those syringes. Congratulations, you've just build a chamber cc'ing tool that would look right at home next to Oldsalt's degree wheel. :wink:
Regarding milling, many say that going too far will kill the flow on a stock head. Increasing displacement seems to be the best and unfortunately most expensive way to raise compression on a flathead.