Determinig the amount of backing for an unfamiliar reel is always a problem.
You must have enough reserve when you catch a big one, but must not overfill the spool to avoid balling you line.
The best way for finding out how much backing fits to your reel i know of is the following:
- stick your fly-line to the spool of the fly reel with tape.
when it is a WF (weight forward) line, make sure you tape the shooting head (the end with the larger diameter). - wind it on the reel.
- when finished tie your backing to the fly line and reel as much backing to your spool till you think the capacity is ok.
- cut the backing with a scissor or knive
- after that strip the whole off the reel.
- tie the cut end of the backing to the reel with an appropriate knot.
- reel the line back in and you will have the proper amount of fly line and backing on your fly reel.