I read the 30° thing in this article

https://health.howstuffworks.com/wellness/food-nutrition/vitamin-supplements/how-much-vitamin-d-from-sun.htm

but it is unsourced

there's mention of Dr Mercola but I haven't chased it down

I'm 32°N, and use Oct-June early-to-mid afternoon (shifts by season) walks torso&legs exposed

to get mine on

and milk

no testing no sick no problem🌞