Re: Level leader question - fly sizes
It really depends on what kind of fishing you are doing; streamers, dries, nymphs - in lakes, rivers, spring creeks?
Most fly fishers use the "X system" to gauge tippet size. A size 12 fly would generally need 4X tippet, which is probably around 6lbs. To properly present dry flies, the length and size of the tippet work together. Actually, to turn over any fly it is a factor of both variables.
I fish rivers. When fishing dry flies to trout in moving water I use a leader constructed like this: 4' 25lb butt section + 1' of .013 (I think that's around 10lb), for there I either tie on 4 feet of 4X tippet and be done, or to taper it longer add 1' of .010 and then the tippet. The idea is to have a stiff butt section to transfer line power get the leader turning, then have a long supple tippet that does NOT straighten out completely. This is contrary to the notion that good casts straighten leaders like a pin. Rather your tippet lands with slack which allows the fly to drift more naturally.