I belive you could follow the directions that James has put together for the Premio at this link. It is an Adobe Acrobat file. If you need a reader to open the file do a google search on Adobe. Adobe has a free download reader.
http://www.fornobravo.com/PDF/Premio_Installation.pdf
So I took a look at what was written and it does not address your question specifically. It would seem prudent to do a dry run without the mortar to determine if it is best to lay the floor tile with the recommended diameter and then fill in the voids of the floor or to tight pack the tile. In either case you need to make sure that the dome will fit around the floor tile. I have a suspicion that if you tight pack the floor (you would still have some gaps but they will fill in with ash over time) you may have a gap between the floor and the dome/vent assembly.