Printing to be viewed at 45deg I have a sort of shadowbox frame. I want to print something (a mountainous landscape) so that it's best viewed when the frame leaned back from the observer at 45 degrees. Is stretching (to 200% height) the best way to achieve this?

I have a model-prop that will stand up straight (-45deg from the backdrop) in front of it.

I am not particularly creative, or close to well-versed in artistic terminology, which is painful when I'm trying to create something.

Terribly sorry for the bad demo but this is what I'm thinking of




Black is the frame
Red is the picture
Purple is the the prop
Orange is a triangle of wood to hold the prop up
And blue is the support for holding the frame up.
Your model is quite clear, although the question is less so. You ask if increasing the height of the image in the background to 200% is the best way.

The best way for what purpose?

If your objective is to maintain an approximation of the original image, the number would not be 200%. 

Consider that the red line extends below the forward lip of the shadowbox. Your measurement point should begin at the intersection of the orange triangle and the red line. From that point, the correct calculation is to use the square root of two times the original height, presuming the 45° mentioned in the question. 

For angles other than 45° it is necessary to use a bit of trigonometry.