Glen, do you have the
Hotech SCA flattener?
If yes, I asked them a while ago about how accurate the 55mm spacing to sensor has to be and got this response:
Quote:
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The tolerance is +/- 0.5mm. This varies depending on the focal ratio of your scope but keep the range within 1mm will be good. Thank you.
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Not sure how much it helps but at least it is a guide how close you need to get. I am guessing that the tolerance is smaller for faster scopes.
By the way, you can calculate the shift using
d = t * (1-1/n)
where t is the thickness of your filter (2mm) and n is the refractive index of your filter. n=1.5 may be good starting guess (glass has refractive index of 1.5-1.6).
This will give you a shift of 0.67mm. Remarkably close to the 1/3 rule 
You do have to increase the shift again because of the sealing glass. Assuming 1mm thickness and the same refractive index for the filter and the glass (to make the calculations simpler), you get the total shift of:
d = (2mm + 1mm) * (1-1/1.5) = 1mm. Double from the manufacturer specified +/- 0.5mm.
I did similar calculations when I removed the AA and UV-IR filters from my DSLR - the spacing was shortened by 0.53mm if anyone is interested.