The answers seem to be slightly different depending on whether you use an angle iron or a brick arch for your oven/entryway transition. If you use an iron, that pushes your vent outward from the dome; at least it did in my case.
Here's a discussion of numbers for the vent floor that I asked awhile back:
http://www.fornobravo.com/forum/f6/h...size-2803.html (Help with vent floor size)
Hope that helps a bit.
George