Thanks to Joseph Gittleman for submitting this how to article.

This is a problem for most angler as judging how much braid and
backing the reel will hold can be a bit of guesswork at times or very
frustrating if you come up short. The method to solve this is very
simple but does take a bit of easy work. and some simple equipment at
hand.
1- spool the empty reel with the amount of braid you want on the reel
2- tie in your backing using a simple knot, nothing fancy needed
3- fill the rest of the spool to your desired lip depth with your
backing of choice
4- Now take off the backing by either reeling onto a spare spool or
use an old line filler spool through bolted and inserted into the
chuck of an electric drill.
5- take off the braid using the same technique, but onto a different
reel or spool
6- now that the reel is empty, tie on the backing you just backed off
and reel it on.
7- tie in the braid, with a good knot this time, and reel it back on
under moderate pressure
8- you now have the reel spooled perfectly to the lip depth you want
for optimum casting.