How many inches to put below the trim on the ceiling

What do you mean by trimline? Are you saying that you are applying wainscotting halfway up the wall and then thinking of trim right at the top where the ceiling and wall meet ... known as crown molding?

Personally, I would put the wall trim right against the edge of the ceiling trim. If you space it, I would think that would visually shorten the room height. Just my opinion, though.

PS ... Other tricks to make a room look as though the ceiling is higher is use vertical stripes on the wall whether it be in wallpaper or paint but try and keep the stripes not overly wide. Also, hang the curtain rod as close to the ceiling as possible and hang long curtain panels that touch the floor ... in other words, no curtains that end at the lower windowsill. If you have two framed prints that you usually hang side by side in a horizontal line, change that to a vertical arrangement - one hung above the other. Just a few suggestions.

I think not more than 5 inches looks best and really depends upon the rest of the elements of the room. ie; if your window casings and floor trim are narrow say 2", a 5" space at the top may draw the ceiling down; if the room is small, trim on the ceiling may be a better; trim on the wall and ceiling might also be a good option. I would suggest doing a mock up (test) using freezer paper to determine if you like the look.

Actually crown moulding can make the ceiling look lower, so test a piece before you commit to it.

Yes!!! Thank you All for giving me your opinons.

Instead of putting trim right at the ceiling, you can come down a foot on the wall and then add a trim! Paint wall under the trim any color you like? Paine the wall above trim same color as ceiling. Then paint your trim whatever color you like! This will make the ceiling appear higher then it is! Also this was done in many older homes too