Definitely don't use the 55 slow jet, as that is way to rich. I also have a 2002 kx 250, and I use the same jetting kit with the same exact jets, along with the blue needle in 3rd clip position.
My bike idle's great. My air screw is 1 and 1/4 turns out, as specified by my manual, and it idles great, and throttle response is great. Try playing around with the air screw until you find the point where it idles good, AND throttle response is good.
Also, you may have to turn up the idle a bit; the idle can be turned up by screwing the screw with the spring on it, inward. Outward will slow down the idle.
I wouldn't really worry about what your bike sounds like when its idle'ing, as long as it actually stays idle'ing, and throttle response is good. If your bike was pinging then that would be bad, but it doesn't sound like it is.