I put enough on to fill the reel. It helps keep the memory of smaller coils out of the flyline. I put the flyline on the reel first and then the backing, that way I know just how much to use. Then I take it all back off and reverse the process. It may take a little longer to do it this way but I hate putting on what I think is the right amount of backing then putting my flyline on only to find out I either have to much backing or not enough.

Rocky