Just asked my brother who's a doctor and he says runners can safely wear shorts down to 40 and cyclists to 45. Suspected it must be in that area. When hiking, the temperature feels 20F warmer than standing, and cycling is more vigorous still, so probably more like a 25-30 degree difference running across, so 45 would feel like 70-75.
Sorry to disappoint you guys but I've talked with a qualified doctor, and the little "60-65" joke's over. I'd thought this was meant to be a sensible forum.