If you post the reel, the test braid you'd like to use, where you're fishing (boat, surf) and your target species, it would help a bunch.
While it might not be the most economical choice, I get my reels spooled at a shop that sells line in bulk. If it's a surf reel, the spool generally gets around 50-75 yds of 20 lb mono backing, then the reel is filled with braid. It's very rare that anything from the surf will take more than 250 yards of braid off your spool, unless your drag is set incorrectly. If it's a spinner that's meant for tuna, I'll have just a single layer of mono put on, and the rest of the reel loaded with tuna spec braid. They can run.
A little pricier than buying a 300 yard spool probably, but
much less headache. Most tackle shops should be able to do this for you.
There's definitely a formula to determine exactly how much braid to put on if you put X yards of mono on the reel first... Just don't have time to figure it out right now.
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