Filling the spool to 90% capacity is recommended. Too much line on a baitcast reel could increase the chance of backlash, while not enough line could limit the casting distance – and the fish-fighting action. For spinning reels, a good rule of thumb is to fill the spool until there’s at least 1/8th of an inch of room from the line to the edge of the spool lip. That will let you use the most line capacity without causing line to spring off the spool and form tangles. The 1/8th inch rule applies to spincast reels, too, though you’ll have to remove the front cover so you can check the amount of line on the spool.