If no one has an answer here, if you can find a copy of - History of British Army Postal Service: Volume III, 1927-1963, by Gould, R.W., and Proud, E.B,, Proud Bailey, 1982. you'll find the answer there (I don't have it but am aware that it lists FPOs up to FPO 1063)
An FPO dealt with units within a geographical area so something from FPO 546 came from a unit near to the FPO. One can't say how near, depended entirely on where it was.
Max