Pretty simple really - I use bezier masks because I *think* they take less render time than cookie cutter effects.

Step 1:
- once in the bezier masking tool window within the P/C button - create a diamond (or square, but diamonds just made more sense to me).

Step 2:
- Make each side the same length (easiest way to do this is to select each point individually and then hold down the shift button while using the arrow keys to snap it to the appropriate grid point opposite its counterpart on the other side of your axis.

Step 3:
- Select all the points of you diamond (square) and then take the creation tool and click on one of the 4 points (will only apply this action to the ones selected, if one was not selected you will then need to click on it separately). this will round the points but you're not finished yet.

Step 4:
- Now individually select each point and move it in towards the center (usually about 2 (Shift+arrow key) movements in. You should now have a perfect circle.

The amount of movement is dependant on what you have your "grid spacing" set to. Mine was set to 64 (highest it can go I think).

Hope this helps some folks out there.

Dave