I have 125yds of 4lb Fireline on my 2000 Laguna (with mono backing). Based on how much the Fireline filled the spool, I think you could put 300yds of 4lb Fireline on a 2000 Capricorn. You could do a rough calculation based on the diameter of the Fireline in comparison to diameters of 4 or 6lb mono (and the stated spool capacity).
One way to to do it and get it right first go is to use the spare spool. You put the Fireline on first and either cut it off at the correct level, or add mono backing when you run out of Fireline. Then take the spare spool off the reel, fit the spool you are going to fish with, and wind all the line from the spare spool onto the reel.
Would be keen to hear how much Fireline you fitted in the end.
Cheers,
Jeff