Well i hear people fill the spool with a certain amount of cheaper line then on top they use the expensive fluro.

I was wondering how much filler line to use before connecting fluro. And what knot to use to connect the lines?

Depends on the size of the reel, and what you plan on fishing for..i.e. for stripers, obviously you'd need more, but for LMB..on a 100 size reel, I usually put about 50% backing..you could get away with less.

You can use a few knots, I use a blood knot, because I never plan on getting down to the backing. You could also use a uni to uni, but it's kinda big.