treebykooba:
I have the speed set to 600. Can I assume this is 60 RPM? how would I know?
I suspect that speed() is the number of steps per second, so 600 would give you either 180 or 360 RPM, depending on your motor type.
To check, try setting the speed to a low figure (say 10) and timing one complete revolution.
10 steps/second on a 1.8 degree stepper will take 20 seconds for a complete rev. On a 3.6 degree stepper it will take 10 seconds.
Once you know how many steps are needed for a complete revolution, divide that figure by 15 to tell you how many steps are needed between each flash of your LEDs. If, as I suspect, that figure isn't a whole number (100/15 = 6.666) you'll have to juggle the number of steps between each flash to average out the decimal part (for 6.666 use 7 steps, 6 steps and 7 steps). Once you can do that, the RPM can be set to whatever speed you like without affecting the timing.