Donghun,
Excellent info! Exactly the help I was looking for! I have the WR2.5. I think all three formats are 0.2667 microns each step.
I entered data in the formula in the link you attached. With my reducer I believe it puts my scope at f/4.8-ish and the ZWO ASI1600MM-C is 3.8 pixel size. Seems, for Red, I got 274 as critical focus so I should try around 300 or 350 step size.