Fishing Line question...how much is enough
Getting into backing is not big deal if stronger than mainline, one reason I recommend braid because of strength to diameter ratio. I don’t run backing on my down rigger reels though, just strait fluorocarbon. You also never need to replace braid backing like you would mono. Only reason people use mono backing is cost. There are calculators to tell you how much backing to load.
A GB said make sure arbor is full. My opinion is a minimum of 300 yds of new mono.
It depends on reel capacity and what you are rigging, rigger rods, dipsys, leadcore or copper. The amount of line on all my setups varies because I want the reel filled to capacity because you get back more line per turn of the reel when it is full. Also remember when you have a big fish on the is taking a lot of line your drag is getting tighter and rather than tighten you should loosen it up and wait until it tires then re-adjust your drag.