Alternative mounting plate for the Helios 44m-4
May or may not fit the other revisions.

Carefully unscrew the four screws at the back of the lens to remove the M42 mounting bracket.

Unscrew the center lens-element. Be careful not to get any dust in. Slide over the 1.5mm spacer ring. Screw back in.
-Alternative, more complicated but perhaps better method below

Use a hobby knife to adjust the screwholes so they fit exactly right. Screw down the rear plate using the original screws. Make sure they don't stick out.

You should now be able to focus slightly past infinity on Nikon F cameras.

This modification is non-destructive and fully reversible. It also features manual aperture control using the ring.

First example picture is taken at f2; the second at f5.6.
(both trough a window in bad light though)

If you're up for it, this video describes how to actually move the optical groups backward, thereby not changing the optical design! It should work on most lenses, and if you need that last extra kick that 30% or so can't get, a way shorter spacer should suffice.
This method requires opening up the lens a lot further though.
https://www.youtube.com/watch?v=FHXMXRhoMNI
Thanks Bhopzzz for bringing this solution to attention :)