[Joe]

I'd say about 30%-40% of the spool should be backing, depending on how full you like your spool.

I know the spool takes 300 yards of 20# mono which has a diameter of .018
If you divide the diameter of the Fireline by the diameter of the mono - then subtract that percentage from 1 that is the % of backing you need to fill the spool.
For example:
300yds X .018 = Full Spool
Say your braid has a diameter of .012 (hypothetically)
.012 /.018 = .66
1-.66=34% of spool backing needed
.34 X 300 = 102 yards of 20# mono backing needed