What size monofilament backing should I be using on my flipping and punching setup with 50 pound braid? Would 12 pound mono work or do I need to step it up to get the knot to work better where the mono and braid connect. Just wanted to know what you guys use. Do i even need mono backing since its a baitcasting reel? The reel specifically is a lews super duty so the spool is braid ready. Thanks

12# should work, but what braid are you using? Thickness of braid will vary from MFG to MFG. 

I use 12# for backing all the time.  I'd use backing even though the reel was designed for braid because of it's cost.  Can top off at least 2 reels with one spool of braid.  Use an Albright or Alberto knot for the connection.

12, 14, 20, I don't think it really matters on backing. Your just taking up space on your spool.

Backing it doesn't really matter. Now if it were a leader I like to try to keep it close to the same diameter.