Remove the rear element of the lens and drop one or more of these inside to push the rear element further back and dramatically improve the focus distance.

Nikon DSLRs have a 46.5mm flange distance, Helios 44-2 lenses are built for cameras with a 45.2mm flange distance, and most adaptors add 2+mm of additional distance, so with two washers it should get you into the right ballpark. You'll need to grind one down if it focuses past infinity, or grind the extra 1mm one down to an appropriate thickness it still doesn't focus further, as usually there isn't enough remaining thread on Helios 44-2 rear elements to put another 1mm in and still screw the element on. Still, it'll get you a much more usable lens.