There are at least three ways to achieve flat bottoms to bowls without the use of external supports.
Using drop out rings will enable you to get a flat bottom of whatever diameter you wish depending on how long you let the aperture drop run.
You can put some dry kiln wash into the bottom of the mould, then firmly press it flat with a round piece of glass. You will need to make sure it is horizontal, so the use of a small round levelling bubble can make this easier.
Grind a flat spot on the bottom of the otherwise finished bowl. It is a good idea to use a two way leveling bubble while grinding. The round bubble is easier to use, while the two way bubbles – two leveling bubbles placed at right angles – are more accurate.