I haven't done any in depth checking in FFXIV, but I do remember from FFXI it was a player stats vs target stats calculation instead of the bracket approach many people seem to believe is in use for FFXIV.

I remember for a long time in FFXI many believed it was a bracket system based purely on the players' stats until some people had enough gear variance and alts to prove otherwise. As most FFs use the player stats vs target stats method it would not be surprising if they use the same system for FFXIV.

However, getting accurate formulas with the player stats vs target stats method is more difficult as we don't know the targets' stats nor if there is a variance on them.