Most everyone I know uses 27# leadcore for salmon/trout trolling. It comes in 100 and 200 yard spools and bulk spools can be found. Filling your reel to capacity is desired because the retrieve rate is maximized. The left over segments can be used by splicing together using a short piece of 30# mono and 2 Willis knots. Some will say too many knots but after quite a few years of doing this, no failures.
I fill my reels like mentioned before. Easiest is to use same sized reel, fill backwards and transfer. If same sized reel is not available I will take the time to transfer between 2 spools and back on the reel using my electric drill. It takes time but the outcome is what I want. On the Great Lakes Angler site someone came up with a reel capacity formula on an Excel spread to assist calculating backing requirements. I have not used it but I’ve read positive comments from others. Go to the following link and mouse down to reel calculator and open up the Excel sheet.
http://www.glangler.com/tools-and-info
Depending on the size of the reel and the length of the segment one can use either mono or braid for backing. I typically use 300’ of 30 or 40# braid(depends what’s on sale) on all of mine >3 colors because some of my reels are smallish size 320 Penns. All setups with > 5colors are Penn 895 size or equivalent. I use that much braid because I don’t want to be spooled by a big fish. Braid is slippery so I put 100-150’ 30# mono between the leadcore/copper and the braid. Braid is more susceptible to abrasion so attaching the board release to mono I think is much better. (yes more knots but…..)
I use a Willis knot for mono to leadcore and a double uni from mono to braid. I put on the leadcore, then mono, then braid and top off with more mono. When back on the reel I use 25’ of 25# 100% Berkley Fluoro as my leader. When it gets to be <15’ I re-tie.
http://www.netknots.com/fishing_knots/willis-knot
http://www.netknots.com/fishing_knots/double-uni-knot/
I typically attach my Off Shores 2-10’ from the lead core. This does not maximize the diving ability and some will let out one to 3 passes before attaching the board. Most important is to be consistent. If 5 colors is hot and I want to turn my 4 color into a 5 color I typically let out 3 passes more than normal. Not sure this achieves the goal but should be close enough. A pass for me is one sweep across the reel.
One ponder-able is how often to change out lead core or braid due to age? I don’t know and have run some of my braid for a number of years. My 40# Ripcord braid dipsys have not been changed for at least 7 yrs and still feels like it would cut off my fingers before breaking. I do cut back a few feet when it looks fuzzy. For the first time I can remember I had a lead core line break mid segment. My recollection was that 6 color was new last year and guessing something other than old age was at play. I have four 2s, two 3s, one 4, two 5s, one 6, one 7, two 8s and two 10s so changing line very often would be a lot of time and money.
Grey Beard