A number of things come to mind - and just about all are covered by the photometrics of the fixture and lamp - Both are usually available for each from the manufacturers.
Like those that can be found here:
http://www.lithonia.com/Library/IESFiles/Default.asp?Path=/Lithonia_Lighting The other factors of lighting design you'll have to contend with are Hot-spotting, Scalloping, Light color. Art is something that is better "washed" from several angles or more depending on the art.... Flat painting or simular 2 directions could work with a wide beam spead - but say a standing sculpture would need 3 or mor different angles.
But to answer your question it depends on the distance of the art from the cieling - a rule of thumb is 30 degrees from the point on the wall that it hang on - on a 10' cieling this is normally about 15-20" from the wall. But as you might imagine from what I was saying previously, there are a lot of variables... Just remember to wave the finger and lisp when discussing it with others - it's required....
