If you wish to use a braid on the main line, you can save some by attaching a few metres of line (backing) before installing the braid. For example, for a spool with a 500 m capacity, you can put in 200 m of line and 300 m of braid, attaching them with a knot.
This optimises the spool fill. This rig also prevents the braid rotating in the spool.