On a long trip VFR, it is not unusual I have to pass my details ten times or so. If the flight plan Departure is ZZZZ with a DEP/Lat-Long note in box 18, what departure field should I give?
“Private airfield near (nearest large town)”
I would give it as the nearest licenced airfield. I would have thought it’s not that important for the enroute controllers to know exactly where you took off, but useful to know roughly where you have travelled from (direction, distance) so they can quickly appreciate the routing you might take through their area. When nearer a private strip destination, you can be more specific.
FIS and ATC keep a record of every aircraft they’re in touch with. Historically this is done on a little strip of paper and includes ADEP and ADES so you have to give them something. What you give them, doesn’t really matter…
The problem with “private airfield near” is that it requires ICAO Level 1 ELP which is not assured south of about 50N 
I would use the ICAO code of a nearby airfield, and keep it consistent from the very first FIR contact.
Actually you could file a VFR flight plan from a farm strip and put down that nearby airfield ICAO code as the ADEP and nobody would find out, but that would be illegal… also that airport might get a phone call if something happened to you.
The original question is a very good one!
