Klaus + Jack, thanks for the input. Should I get the shorter helicoid instead of the longer one? I attached pics to confirm with you before buying.
Today I experimented with a thin M39 to m4/3 adapter and my CRT combo. The m4/3 mount is too small for the M39 to go through, and the end of the long M39 stopped at the opening of the m4/3 adapter. So, the distance between the mount of GH5 and the end of L39 thread of CRT was about the thickness of the adapter (a bit less than 2mm, I assume).
The combo turned out to be impossible to focus anything: near or at infinity. I guess the CRT was too close to the sensor, well past infinity.
The combo described above should correspond to the 12-19 helix, M42-NEX adapter and M39-M42 combo. Considering that the flange back of Sony NEX mount is about 2mm shorter than that of m4/3, and the M39 thread of CRT could pass through the opening of the mount, the 12-19 helix might be too short to focus to any portrait distances, perhaps even at infinity.
On the other hand, when the CRT (without the retaining ring) is mounted via Nikon BR-15 and F-to-m4/3 adapter, the lens will focus at around 1:5. So, 35-90 helix could be too long and only for even larger magnification.