I need to space 2.413mm circles along an arc, exactly 6mm apart from their centers. I can't use Fit to Path as It doesn't have an option to set the exact spacing. I can easily do this on a straight line using Step and Repeat.
You could create two circles, then Blend along the path. Blend has an option for setting the spacing. You could then break apart the blend. Doing it that way would mean that the "circles" produced would be Curve shapes, not Ellipses.
If the arc is a circular arc, then you might accomplish this by rotating/duplicating. That would require finding the center of the arc (to use as the rotation center) and calculating the angle that corresponds to 6 mm spacing on that particular arc.