7070 ok first are you using. a bellows or extension tubes with helicoid. I like a bellows myself. because I make changes on the move. 7070 tell me if you have any adapters at all.
1 I use nikon PB6 bellows so your nikon D7200 mount on that bellows.
2 lens end of bellows you will need a nikon F mount to m42 Adapter..
https://www.ebay.com/itm/Nikon-F-Le...72494109252. here is a link for the adapter
3you need and m42 to m39 adapter...NEW M39 to M42 Screw Lens Mount Adapter Step Up Ring Fit for Pentax M39-M42 Cam.. here is the link for this adapter.
4 ok so if you want to go by a helicoid here will be the setup focusing helicoid...
https://www.ebay.ca/itm/M42-Lens-to...AOSwGYVW-jv7 here is the focusing helicoid
5 m42 extension tubes...
https://www.ebay.ca/itm/M42-Macro-E...AOSwgeBadBT3 buy at least 2 sets of these tubes.
6 m42 to rms adapter...Adapter RMS Microscope objective to male M42 X1 adapter Flat without flange... I prefer this adapter later you will understand why. see with this type of adapter you can counter sink your objective.
7 I would recommend you buy a m5 objective. I will see if I can find rays link on
ebay.
8
https://www.ebay.com/itm/New-Adapte...AOSwNSxVekvZ here is a M4 which I have and surprisely it works very well. hope this gets you started