FSharp - 153bytes
let t(s:string)=
let c=s.[2..]|>Seq.map string|>Seq.reduce(+)|>float
match s.[0] with
|'B'->sprintf"Ft%.0f"(ceil(c/2.))
|'F'->sprintf"Bo%.0f"(c*2.-1.)
Psytronic
- 161
- 2