Measuring for your board mounted valance is quite easy. First you need to determine if you are planning on hanging panels underneath the treatment.
If no panels are to be hung then you need to measure from point A to point B. Point A is where you want the valance to start and point B is where you want the valance to stop. That’s it – you’re done.
When no panels are to be hung under the valance, it is best to have the valance look form fitted, about an 1” wider than the actual window or window molding (if you have molding).
If you do plan on having panels under your valance, you will need to decide if you want the drapes to hang in front of your window or stacked off the window.
If you choose to stack the drapes off the window and you have the wall space to do it, then you will need to order the board mounted valance wider to accommodate the drape width.
This will give of course make your window appear wider than it really is, which many people like to do. A standard 50” wide rod pocket panel when dressed in at about 2-2 ½ times fullness will take up 20-24” of wall space for each panel.
Keep in mind you will want to start the panel at the windows edge (not the molding edge). So if your molding is 3” in width and panel will take up let’s say 22” less 3” (molding) = 19” additional width on each side in order to stack the panel off your window.
When hanging drapes under a board mounted treatment you should order a 5 ½” projection so that you have enough clearance to mount the valance over the drapes. If no drapes are to be hung, then a standard 3 ½” projection will suffice.