Hi Waldemar,
I think at the level of precision that you are talking about, tenths of a mm, you might be right about there being many variables. But the basics are pretty well defined - focal ratio being the key. Certainly other manufacturers and sellers seem able to provide detailed information. We have a situation here where on the USA version of the ES website they suggest 55mm back focus and on the European version they suggest 65mm! There's definitely something wrong there.
Fortunately, someone very helpful from ES USA did respond quickly to my enquiry - see below.
'I consulted with one of our optical engineers to confirm: The measurement starts at the rear flange (flat surface) of the corrector. NOT from the CCD M24x0.75 plate -- take that off, then start the measurements.
The M42x0.75 plate adds approximately 3 mm.
The backfocus is 55 mm. There is an additional 2 mm either side where the correct is acceptable.
You cannot judge T-rings by their total thickness – because that is not the amount they add. They have threads on one end that screw onto the M42x0.75 adapter – so those threads don’t count. The T-ring also have a bayonet (or other) mounting system that, when coupled with the camera, subtracts from the T-ring’s overall measurement. All those variables have to be accounted for in creating your system.'
It is puzzling that ES do not support some of their products with clearer information. No guidance at all comes with this product. A similar reducer supplied by Willams Optics comes with very detailed information about back focus and other parameters.
Thanks for suggestion of the variable extension tube - I had thought about that approach but unfortunately to get the correct backfocus with a Nikon DSLR I am going to have to go down the low profile T adaptor and very thin extension ring route.