Here are the few formulae you need to know it all.
Put them in a spreadsheet, or write a simple program, together with lens data (can be found at Zeiss' website).
Magnification = Extra extension / Focal length
Aperture correction factor = 1 / (Magnification + 1)
Shutterspeed correction factor = (Magnification + 1)^2
Correction in stops (EV) = log(Shutterspeed correction) / log(2)
or,
Correction in stops (EV) = log(Aperture correction) / log(sqr(2))
These formulae are correct for symmetric lenses. Asymmetric lenses require a little more (telephoto), or less (retrofocus) compensation.
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To calculate the compensation (and that only) for asymmetric lenses, multiply the focal length by the pupillary magnification of the lens:
Pupillary magnification = diameter exit pupil / diameter entry pupil
so
As.Focal length = Focal length * Pupillary magnification
and use As.Focal length instead of the true focal length in the above mentioned formulae.
The clearsight tables do not do this, though the difference between these 'plain' and 'asymmetry corrected' values can be surprisingly large.
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Remember that the focussing helicoid is providing extension too.
The clearsight tables give one compensation value for each lens-tube combo, but since you can use the focussing helicoid too, they should show the upper and lower limit of a range.
You can deduce the amount of built-in extension using the above formulae and the minimum focussing distance.
But you can also measure the differences in physical length of the lens set to infinity focus and minimum focussing distance.
