How much line to put on baitcast reel?

I am using Shimano's Bantam Curado Baitcasting Reels, regading the amount of line to spool up on one it appears to be pretty much a waste of money to put much more line than the distance one is able to cast, trolling situations excluded from this conversation, with that one needs to put some type of filler on the spool so that the spooled up line extends to the recommended point on the spool for optimal casting. As it will take some trial and error to determine the amount of filler to put on the spool the filler will need to be somewhat easy to remove yet be anchored well enough to the spool to prevent slipping. The question is what to use for filler?

Here's what I do: take your old line and use it as a backing for the new line. Like if it holds 165 yards of 14 # test, use 75-80 yards of old as backing and the rest new.

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