just finished a penthouse condo with 18 ft, curved ceilings. Ceilings also had 2 copper tube and steel braided wire support frames running the length of the curved ceiling. Typical sq.ft measurements of the ceiling and typical time per sq.ft of ceiling went the way of the dodo as soon as I took a look at it. So I visualized in my mind how I would do it and roughly how long it would take me, then tack on a few hours for good measure.
My edumacated guess was pretty spot on.
For a typical 10ft or 12 ft ceiling I simply estimate the room same way I would an 8 ft ceiling. Just add the extra sq.ft to wall area. I find there is no real noticeable time added for a higher ceiling. Ladder movements are the same. I have to cut up there regardless of if its 8 ft or 12. Just the extra time and paint, at the same sq.ft. rate, for the taller walls.
Side note. I rarely, if ever, subtract doors and windows from wall area calculations. The added time to cut around them is covered by the sq.ft rate it takes me to roll the walls. More or less.