Dngallagher I strongly disagree with your recommendation of the Xit XTETN Auto Focus Macro Extension Tube Set for Nikon...
I actually purchased this product and it is literally a "piece of crap" if anyone wants mine I'll gladly sell it for ten bucks (plus shipping)
My Xit XTETN Extension Tube set will only work if you constantly jiggle the lens/extension combo so the contacts mate up...
And one of the tubes is completely defective i.e. will not activate auto-focus or metering... virtually worthless... or maybe $10
However that said I ultimately purchased the Kenko Automatic Extension Tube Set DG for Nikon
https://www.bhphotovideo.com/c/product/375238-REG/Kenko_AEXTUBEDGN_Auto_Extension_Tube_Set.htmlwhich is a #1 Top Seller on B&H and for compelling reasons... the build quality is an order of magnitude better than the Xit XTETN junk.
The Kenko works flawlessly on all my Nikon bodies... and every lens I've matted it to... Penny wise and pound foolish comes to mind...
btw, Kenko is the same as Tokina and they're well known for quality...
John Martin I use the Kenko Extension Tubes on my Nikon D7100 and it is a very good match...
And as with all extension tubes, attach the tube to the body first...
Caveat! Do not for one moment think about using any Extension tube set on focal lengths longer than a normal lens...
If you try to attach a heavy 70-200mm lens you will absolutely place excessive stress on the tube mounts (they are not metal)
As such the mount will most likely fail catastrophically... You've been warned!
However if using the tripod foot/collar mounted on a stable tripod you might get away with this combo, but are you willing to take that risk?
Hope this helps, or is at least food for thought...
I wish you well on your journey John Martin...