I always fill about half the spool with mono backer. I only put on about 20- 60 yards of good line on each reel. On my flippin sticks I put on even less. It just depends on how far you cast each particular reel. Do you have several reels or just one? To me it is application specific. On my crank setups I have a lot more line on then my flippin sticks or even the ones I know I won’t cast more than about 50 yards. There is no reason to spool up 125 yards of expensive braid, when you are going to only use 1/3 of that. Same with fluorocarbons...they are not the cheapest lines on the market either. I usually use 10 or 12lb mono for backer...all the backer is used for is to take up space on your reel, it serves no other purpose really. Some guys will tie their backer to the main line, but I don’t. I will either use a thin piece of tape to keep the backer line from moving, or I will wrap the backer into itself to hold it in place, then cut off the little tag that hangs out. Either way, I sill tie my main line to the spool, not the backer. Hope this helps and I’m sure a few more guys will have some input.