Seriously, I can't think of any of my friends who regularly spend time outside with their shirt off each week, which I'm pretty sure is what is meant by "minimal clothing."
I think that's a bit of an underestimate. Short sleeves basically expose only the skin on the top of your arm from the elbow down. That's a lot less surface area compared to your full front and back, shoulders, and neck. Your chest/shoulders/upper back will also be receiving direct, unbroken sunlight, while your arms are probably blocked a good deal when they are at your side by shadows.
If it were that easy I don't think deficiency would be so prevalent. I agree that it's almost impossible not to get 30 min of partial exposure.
SPF means "sun protection factor". SPF 10 is not "completely exposed" it's actually a very high level of protection, it means that it will take 10x as much sun exposure to lead to the same amount of UV-B reaching your skin, more or less.
SPF 10 is 90% covered, and cotton T-shirts are usually higher than that. You'll find that you can see visible light through a T-shirt at around a 10× reduction too.
is there any problem with going outside before you have lunch, taking off your shirt and running a mile? this should allow for at minimum 5 minutes, at maximum 15 minutes of sun exposure.
In the US, there are laws against this kind of thing, although they typically exempt nearly half of the population. Worse than the laws, though, are the social sanctions.
i assumed the half of the population who weren't allowed to take off their shirt would wear some kind of minimal sports covering. even an a-shirt or something similar would still expose one to a sufficient amount of vitamin D, perhaps with a few more minutes exposure.