When you're measuring for window treatments, think about how deep you want your valance to be. You don't want it too shallow from top to bottom since it will look long and narrow. If it's too deep, you'll cover up too much of the window or it could look top-heavy.
No guesswork needed! To get it just right, decide where you want to place the top of the valance. Measure from there to the floor or sill.
Divide that measurement by 5 or 6. The number you get will tell you how deep the valance should be to provide a window treatment with good proportions.
It's as simple as that!