bigfish, I don't know the capacity of your reel nor do I know the bulk size of 18# leadcore. I use a okuma #45 with about 50 yrds of 25 mono then the 10 colors of 27 lb leadcore. I can only hope this helps. You do not need a large amount of backing because you will normally not even get to the backing, but if you do this should be sufficiant.
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Eat, Sleep, Hunt and Fish (The Devil is in the details)
Dan Duncan