That window's kind of small for a valance. Do you have room above it to hang it higher? If so, just make it 12" and hang it high as you can. It shouldn't be covering more than 1/4 of the window.
I think a Roman shade or short panels with tiebacks might suit the window better.

hi shadylady, you are so right! I'm not a seasoned sewer, but have made valances for my windows. These are for my
mil and a straight valance would not look good! she has the short panels with tiebacks now. Sure glad i posted this :)