I'm putting in new cornice for a room we've built but the ceiling slants. I realise that on a normal flat roof the overall angle is 90 degree and so the cuts are 45 degree but the angles from wall to ceiling aren't 90 degree. How can i work this out?

initially i thought that if the highest angle was something like 85 degrees i would half that to give me the angle of the cut but in actual fact the cutting angle would have to be more than 45 degree.. is that right?


More...