The first few times you do it you may have to experiment with how much backing to put on. It doesn't have to be exact, nor does it have to be rocket science. How much backing you put on will be determined by how often you plan on changing your main line. If you're one of the guys that does it every few trips you reel will be spooled mainly with backing. If you're like me and go the whole year without changing, you'll probably end up somewhere in the 25/75 ratio. After you get it where you want it, you never have to mess with it again. When you spool up you take it down to the knot, cut it, retie, and fill the spool back up. Get it close to where you want it, and then you can adjust it later.