I'm trying to get a good estimate of the risks associated with sun exposure.
I'm assuming things scale linearly (the UV index units, which need a name ("UVees"?) are linear).
This may not be a good assumption, but it seems somewhat likely that it would be within the range I most care about (significant exposure, but not enough to get burnt -- if I'd get burnt, I'll certainly cover up or wear sunscreen).
I know sun exposure is a major source of skin cancer risk, but I'm not sure how much of that is coming from people who have tons of exposure, and how much risk a moderate level of exposure actually brings.
Pretty sure this is incorrect. It's not the damage that causes cancer, but the failure of the body to heal/repair it. Such failures can be caused for example by you being very old, and therefore healing slower, or by getting a sunburn (= too much exposure in a short time, overwhelming repair capability).
I think the most important thing here is that things scale very much not linearly.
See also this, which argues/claims that more sun exposure (without getting sunburnt) actually leads to less cancer than getting less UV total, but with sunburns: