Use trigonometry. You need to know the height of your art to the height of the ceiling...you have the angle (up to 35 degrees) so you can figure out the distance the light needs to be from the wall.

But once you calculate all that and pick the perfect spot, you'll discover you have a stud there that prevents placement... :-)

Typically 1 - 2' away is good for a standard 8 - 9' high ceiling.

http://inspectapedia.com/BestPractices/Figure5-23.jpg

@mmilos- I suppose you can use the cosine function here but if I use values that range between 1 and 35 degrees my many answers wouldn't necessarily tell me which is my best choice for distance (although many results for distance can be eliminated for practical reasons or fixture specs). Keeping it simple and not factoring in other fixture ranges for lumens or K- values I just wanted to know a good rule of thumb. It's not like I'm doing the lighting layout for the Louvre here but I don't want to have to cut through another one of those wooden things in my ceiling that always seem to be right where I want a can light... ;-)

@ Fred S - Thanks a million!!! My dilemma now has a fork in it and is completely resolved. I won't have to worry about the extra work of patching, sanding and painting sheetrock.