I'm sure you could get lots of different answers based on the setup, but you only need enough backing to allow the braid to dig into it so the braid won't spin on the spool. How much braid you put on is a different story. I usually use a 150 yard spool. Yes, I tie the braid to the mono, and the mono to the spool.
Somtimes it gets tricky trying to figure out how much backing to put on the spool so that the 150 yards of braid fills up the spool to the ideal amount. I stole someone else's idea of tying the braid to the spool first, reel in the 150 yard spool (or however much you want to have) then tie the backing to the braid and fill the spool all the way to where you want it. (I like mine about an 1/8" from the lip on spinning). Now you take rod number 2 (which ideally has the same reel, or same line capacity, and you reel rod 1's line onto reel 2. You end up with backing on the spool and 150 yards of braid filled 1/8" from the lip. No waste of line, and no under-filling the spool.