Best thing to do mate is (since you've got a spare spool), firstly wind on all the braid. Then tie on the mono backing and load that over the top, until you reach about 3mm under the spool lip. Then cut the mono and take the spool off. Put the other (spare) spool on to the reel and tie the mono that youve just loaded on to the other spool. Now wind the mono and braid on from the first spool. As you wind it on you'll first load the mono, then reach the braid. When you get to the end of that it'll be left at exactly the right level
Also, when your loading the braid originally, place it in a bowl of water. This'll help it load flat, bed in well, and not twist.
P.s. Load it tight too. I usually hold a towel between my fingers and run the braid in a couple of loops through it to keep it taught.