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With long johns, I’ve done from a few degrees below to about 8-10. I like wearing long johns even up to nearly 10 degrees though. Wicks sweat away and feel nicer overall to me anyway. You can basically unzip the whole side leg though, poppers keep it together, which really helps cooling. I would wear them on their own when above 10.
Not sure if that helps, and everyone’s different when it comes to running hot/cold, and if you prefer to be a bit on the chilly or warm side of comfortable.
Paramo Cascadia - https://www.paramo-clothing.com/en-gb/explore-range/product/?pk=DC2EC91F-F3F3-4415-9949-D46BFE7B7BE5&attributes=5A01410B-7BF6-46CC-8A97-FFEA0A577F6E
They are great for exactly what you’ve described in my experience. I tend to wear long johns under them.