At my current location, I don't have a recent 650 but according to 3.5.5 of the 98 version it states: In lieu of annular plates, the entire bottom may be butt welded provided the requirements for annular plate thickness, welding, materials, and inspection are meet for the annular distance specified in 3.5.2.
To me this means that even though you don't have annular plates, the requirements would be the same for edge distance between an annular plate weld and the vertical shell weld, and therefore the floor plate weld would require the same distance as an annular plate weld.
Regards,
Gerald