What angle to cut at using mitre?

[kongkong]

//media.diynot.com/188000_187463_61504_85395419_thumb.jpg

I've got this oddly shaped cornice that I need to do a 90degree internal angle with on a wall. Unfortunately I can't cut it on my mitre saw as it is seen on the wall, it needs to be shifted 45 degrees so it is flat.

What angles do I need to cut so that at the corner the two pieces fit in after I've rotated it 45degrees forward? I'm racking my brains out trying to work it out but seemingly getting nowhere.

Thanks in advance.

 

[Porque223]

http://www.woodweb.com/knowledge_base/Crown_Moulding_Miter_and_Bevel_Angle.html

 

[pinenot]

You can't be sure the walls will be at 90 deg to each other, but they will be close, therefore the angle you ask would be 45 deg. However what your really asking is how can you cut the angles down from the ceiling/wall when there is no full reference face to cut it from. What are these covings made of??...pinenot

 

[foxhole]

//media.diynot.com/188000_187463_61504_85395419_thumb.jpg

I've got this oddly shaped cornice that I need to do a 90degree internal angle with on a wall. Unfortunately I can't cut it on my mitre saw as it is seen on the wall, it needs to be shifted 45 degrees so it is flat.

What angles do I need to cut so that at the corner the two pieces fit in after I've rotated it 45degrees forward? I'm racking my brains out trying to work it out but seemingly getting nowhere.

Thanks in advance.

What's odd about it?
Coving is cut at 45 degree thru two planes.