I haven't done this on everyone of my saltwater reels but it's a good idea to include 25' of 25# mono between the 30# dacron backing and the line.
The point of this is to have some stretch behind the line to help keep the tippet from popping from fly line drag.
Once all of your line is out, there's a tremendous amount of pressure on the tippet from the mass of the fly line being dragged through the water. On top of that, many people naturally tighten on the reel's drag when backing is melting off their reel. Wrong !
Having a stretchy mono section behind the line helps to protect the tippet when there's a full fat fly line being pulled sideways through the water.