I normally am a straight fluorocarbon guy but after hearing lots of good things i want to try braid to leader on my casting setup. I am wondering what size line i should use for the main line as well as the leader? It is for a 7ft medium heavy fast action rod that i use as an all around type combo. I normally throw spinnerbaits, chatterbaits, jigs, swim jigs, and light t-rigs all on the same rod. It was previously spooled with 12lb flourocarbon, which seemed to work well for me. 

My general purpose MH/F rig is spooled with 40# Sufix 832 with 12# Yo-Zuri Hybrid as a leader...just for comparison's sake.

My MH/Fast rods are spooled with 30 lb Sufix 832.  I use 12 lb mono leader on some, no leader on others. 

30 lb Sufix 832 with a 12 lb fluorocarbon leader.  I don't always use a leader.  Often, I can get away with just braid.  I usually only use the leader if I am in fear of abrasive underwater objects.

I match the diameter of the braid to the lb test leader I am using. It really helps in tying the connection knots - at least for me. Most storefront websites have the diameters listed. I like Smackdown braid so if I need a 12 lb leader, I match it to 40 lb braid.