I had the same question with my instructor on my first cross country, we took off early AM in the morning and the Valley we were flying down had low morning cloud. so we took off in CAVOK, expected to Arrive in CAVOK but for 90% of the route i was not in sight of surface.From on top I'd say there was maybe 200-300 ft or so between valley floor and the base of the cloud I said was it legal ...my instructors response was ... "yes, but it'll be really interesting if the engine quits"from then on i was trying to visualise the valley floor under the cloud.... where was that farm?..... how narrow is the valley here?