Don't forget one other thing - well known to piston rotary pilots in Yorkshire....
If you are downwind of major power stations with cooling towers that emit several hundred tonnes of water vapour per hour you should pull full carb heat, whatever the conditions of the day dictate, re. the icing temp/humidity graph. The steam is visible, but the plume of transparent water vapour at near 100% saturation for a few miles downwind is invisible....