I am in the process of remodeling a room for my recently moved back in daughter.To speed up the job I used prefinished panelboard.Because the house is older and "settled" the resulting gaps at corners,due to the room not being square, preclude the use of the plastic trim/moulding commonly used with panelboard, the gaps are too wide.I am using the foam/polystyrene type simulated wood trim. because of the gaps I have to run a vertical peice of trim from baseboard to ceiling to hide the vertical corner joint.How do you join these three angles at the ceiling corner.I am using(not sure what it's called)trim w/a 90 deg hidden corner and a 1/4 round concave face.I can't seem to figure out how to do this.How to cut the trim so all three mouldings fit together at ceiling corner joint??? Thanks for any help!!!

Too late now but you should have scribed your panels in the corners to fit.

A picture would help, but if I'm understanding your question you cut two 45s on each side of the vertical piece (it comes to a point) then fit your horizontal molding to each side.

I'm not sure this is the right answer, a picture would help.
This should have been posted in Handyman.

You can cut the angles with a miter saw. A powered miter saw will be a lot easier, but a manual saw and miter box will work. To get the angles right, try with some scrap; it is hard for me to describe the process in words here.

It sounds like you have crown molding or cove molding at the ceiling and your using cove molding in the inside corner between walls. I usually would just use a small trim on the inside corner and just but it up to the ceiling molding (but as you said your inside corner gap is too wide to do that). I'm trying to picture a seamless splice between a piece of cove molding used for an inside wall corner and the ceiling cove molding joint. Could it even be done? Maybe just use a square block in the corner and but all 3 pieces up to it.

I practiced with some scrap , the angles don't work out w/o compound cuts. So in the interest of speed will go with the block as all I have is a miter box. Thanks for the input!