How do you figure IPM?

I have stepper motors that say 200 step per rev. top speed is 24,000
pulses per minute which equals 120 rpm. My question is how do I
figure IPM? Im going to use 1/4-20 threaded rod. So I figure
20x200=4000 step per inch. So heres where im lost I need to the
formula to figure this out. ( I'm not a math wiz as you can all tell
by now! LOL) Can anyone simplify this for me? I'm sure its simple.
Let me know if you need more info...