I usually just lurk, but I can’t resist. The button is your
"safety factor". You can have as large a button as you want.
Actually, the bigger the better. You can reach a point where the
size of the button becomes ridiculous and wasteful (and dangerous
if spin casting), but that’s your call. The size (weight) of the
button should be at least the same as your pattern, if not a bit
more. Your “draw porosity” is a result of improper sprueing.
Sprue the heavier portions of a casting and any other area that
may have a hard time being “fed” and you should have a
successful casting. You cannot “oversprue” or “overfeed” a
casting. Any time spent adding a sprue “to be sure” is better
than recarving a pattern. The button is there to push the molten
metal and to be the last part to solidify. All the contraction
of a button in a properly sprued pattern will be from the exposed
side of the button - not from the invested side. Draw porosity
comes when a pattern is improperly sprued and a heavy unsprued
invested part cools (solidifies) after a thinner
sprued invested part has already cooled to solid.