Alan,
If that 200 f/4 is the AI version that takes 52 mm filters, you could use a 52 mm reverse mount filter adapter to put your 50 mm on the end of it and get 4:1 magnification (or tape the two lenses together instead of using a reversing adapter). Focus with the 200; you might get vignetting.
Mike,
If you're looking for some ideas about tools for close-up/macro photography, this Nikon sales manual is a good reference: https://www.pacificrimcamera.com/rl/00595/00595.pdf. I think it came out a little bit before your FG-20, but it still has a lot of relevant info. The PB-6 bellows still works with my Z8, 55 f/3.5 and f/2.8 Micro-Nikkors, and enlarging lenses.
If that 200 f/4 is the AI version that takes 52 mm filters, you could use a 52 mm reverse mount filter adapter to put your 50 mm on the end of it and get 4:1 magnification (or tape the two lenses together instead of using a reversing adapter). Focus with the 200; you might get vignetting.
Mike,
If you're looking for some ideas about tools for close-up/macro photography, this Nikon sales manual is a good reference: https://www.pacificrimcamera.com/rl/00595/00595.pdf. I think it came out a little bit before your FG-20, but it still has a lot of relevant info. The PB-6 bellows still works with my Z8, 55 f/3.5 and f/2.8 Micro-Nikkors, and enlarging lenses.