Someone was asking similar questions on the garmin forums, that wasn't OP was it?
I suspect you could use the lux reading to do this, but I feel like the correlation would be inexact. So would make sense to use a safety factor. I propose a factor of 2.
I'm pretty sure that if I get skin cancer, it's going to be because of my forearms. The worst sunburn I have ever had was on the back of my hands, I even got blisters. So if someone rides a bike like I do, the watch is likely to be at a very good angle.