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Michael, let me try to explain why I think there is confusion. There are two things that affect the distance:
1. filter is glass and has a different refractive index from air. This will shift the light beam roughly to about 1/3 of the glass thickness. So, for a 3mm thick filter glass you will need 64mm distance instead of 63mm and this can be achieved with a 1mm spacer.
This is what everybody is talking about.
2. Physical thickness of the filter mounting. I think this is the point that most of people did not think of. Usually filters are inserted into filter holders/wheels and do not add to the distance between the flattener/sensor. However, in my case (and probably yours as well) the filter screws in between the flattener and camera and this will extend your existing distance. If the filter mounting is 10mm thick you suddenly have 63+10mm distance and you need to shorten it by 10mm to achieve the optimum 63mm.
(if you include the effect of the glass as in point 1 above you will need to shorten the distance by 10mm - 1mm = 9mm. Of course the numbers are examples and you have to measure your gear).
So, you have two choices:
1. Mount filter inside something so it does not extend the distance.
2. Mount the filter before the flattener. My flattener has thread at the scope side which matches 2" filter threads and this is where I placed the filter.
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