Luckily I'm dark enough to never have burnt, unluckily that means I need more exposure.

Interesting. So 20 minute cycles over 2 hours is probably better than continuous 1 hour exposure. Not surprising, but unfortunately inconvenient from a scheduling standpoint, given that the peak time for D synthesis is supposed to be noon which is during most people's workday. I kind of thought this might be the case and try to mimic cycling by flipping around frequently.

(That said, the noon people might be wrong, longer exposure over less intense evening sun might be better and intense noon exposure).