You don't need the greenboard. You can put the cbu directly on the studs. You may want to put up something like plastic or tar paper and then the backer board, depends on if this is a shower, too.
Concensus seems to be 1/8 -- 1/4" gap. Does the tub have a tile flange? If not, then you may want to do something like Schluter's Dilex-AS trim piece instead of caulking it. (Well, you still caulk, but it is hidden behind the Dilex trim - easier to make it look neat, if you like the look at all!). Suggest you look at www.johnbridge.com for tiling info, too. Alot of active threads - you can learn a bunch from them.