I took some night photos of the illuminated Clifton suspension bridge at Bristol many years ago on 100 ASA Fuji chrome and 20 secs at f8 gave very pleasing results. I'm not saying that's the optimum exposure, and of course the actual light levels are as long as the proverbial piece of string, but it might be a starting point.
Good luck!
Steve
i use the pinhole designer to figure my pinhole exposure times. it is good because it builds in several different reciprocity times if you choose.
i plugged in fp4. because it is set up for pinhole use the largest aperture i could choose is f22. take the meter reading at f22 and read off the chart. you will have to manually adjust for larger apertures.
You can also take a look at Ilford’s data sheet on FP4 and look up the reciprocity characteristics of the film (under the headline – making long exposures). I would also advise you do decrease development of this roll by around 20 percent.