backfocus distance with filters. How to calculate?

[FrancoRodriguez]

Greetings people! I have a CLS CCD filter. It's 7.52mm thick including the housing. The filter itself is 1.1mm +- 0.002mm according to the manufacturer, with a refractive index of 1.5. Does anyone know how long the equivalent extension tube would be for backfocus measurements? I vaguely remember Snell's law from way back yonder but not sure about anything these days!
I saw on cloudy nights that astrodon filters (which I also have on another system) the equivalent distance is 1/3. IE, a 3mm filter adds (or subtracts?) 1mm of backfocus distance. I'm pretty skeptical about that. Any thoughts would be greatly appreciated! PS--Experimentally determining the correct backfocus distance has been hard for me because I seem to have introduced complex distortion/astigmatism due to an unknown factor, possibly an imaging train issue.:(
Thanks in advance!:help:

[glend]

The "Rule of 1/3" is a good guideline. In my experience. And don't forget to add in any cover glass on the front of the camera sensor chamber as well. Most cooled cameras have a cover glass. Also remember that Reducer/Correctors are usually measured from a certain point (not necessarily the last glass surface), like the beginning of the threaded section at the rear. Notes with Reducer/Correctors usually advise where to measure from.
For those that love to find fault, note that I am not suggesting you count in the sensor cover glass (bonded to the sensor) as this is usually considered the sensor, and is very thin in any event, like 0.5mm.