I believe that chart is FBV to ROH not Fire.
I normally skate on a 3/4 ROH. When I switched to the Fire ring I went 3/4. They did not feel the same or even close. I had to drop down to the 5/8 Fire to get the same grip I got with the 3/4 ROH. So rule of thumb when switching to a Fire ring, drop down (aka go sharper) to maintain comparable grip to your ROH.
Probably been mentioned. The Sparx Fire is nothing like a Blackstone FBV. It's closer to Blademasters version of the Flat Bottom.