I don't approach a spit any different than any other bird, it all depends on initial situation on how I go about destroying it.
the tactics should be basically the same no matter what you are facing, get some offset on the merge so by time you pass each other you are already in your reversal and line up onto his 9-6, if you see he is pulling hard, drop throttle and just pull into him, unless you have a good E advantage, then you wont be able to pull in fast enough...of course this depends on what you are in.
initial E states is very important in deciding what to do, you gotta learn to judge your nme's E...from there it really becomes easy on how you react....
example
your in a slow 51...say 250 or so....spit comes diving down with a massive closer rate you know his E state is much higher then yours, wait until he is about 800 out before you start to maneuver, a fake split S will make him drop his nose even more picking up more speed, making it much harder for him to track you.....try to get below their nose and then make your move, he wont see you do it, and by time he sees where you went, he is playing catch up.
the slower plane most often will turn tighter then the faster plane, very few exceptions to this rule....
don't know if that is helpful, I could look for some films of my merges with spits.