10540
Really for any serious flight you need approved anti/de ice. I scared myself silly years ago in a Saratoga with a pre forecast front moving west to east and me flying north to south from Scotland.
It was just the situation you mentioned happily on top at FL6O below CAS with freezing levels well down into the terrain below.
Staying below airways the aircraft entered cloud and iced up forcing me to climb.
Eventually breaking out into final blue at FL120. Yes in CAS but having informed ATC of the problem.
That was in the days when you could speak to a met briefing guy who told me that I would beat the front if I was off before X time.
FL120 was it! No more climb covered in ice. Not a good situation. Glad when I cleared the frontal system down near Liverpool
Suppose thats what they call experience
Pace