I had some experience making and measuring pinholes before I made a pinhole cap for my M42 bodies, so I got pretty good results. One limitation is that a flat cap will have a projection distance close to your flange-to-film dimension, which is usually a little long for 35 mm pinhole -- on M42, for instance, it's 45.6 mm, and a 35 mm pinhole camera works better at 15-30 mm projection. Now, your Fed (or any M39 LTM body) has one of the shortest flange-to-film distances going -- thirty-some mm, as I recall. Should work quite a bit better than the one I had on M42.
Load with Tri-X and make up a batch of my Super Soup, and you can hand hold in daylight, even as dim as cloud shadow (three stops off Sunny 16) at EI 5000-6400.
For a single 35mm roll in a stainless tank:
6 ounces water
24 ml Dektol stock solution
8 ml HC-110 syrup (or 32 ml stock solution)
1 g ascorbic acid
1/2 tsp washing soda (sodium carbonate monohydrate)
2 g potassium bromide (optional)
Water to make eight ounces
Develop fifteen minutes with vigorous agitation every 30 seconds. Tri-X 400 from the 90's will give EI 6400, Tri-X 320 from the same time frame is good for 5000; grain is obvious, but tolerable (not much worse than a 1-stop push in Parodinal) and contrast seems near normal.