You need to know what fabric you want to use so you know the width the fabric is to figure out how much you will need to purchase.

ordinary fabric is 44 or 45 inches wide. Most home decorating fabric is 54 inches wide. I wouldn't bother trimming the width of the fabric, just use is as is. If you're using a 4 inch rod. and three inch hems, then you need to add 14 inches to the finished length of each panel, multiply by the number of panels, and divide by 36 to get your yardage.

The description of your valance is unclear. If i interpret it right, you're using a 4 inch rod and you want a dust ruffle of one inch at the top. The hem is 1 1/2 inches double thickness. So, 15 inches, plus 8 for the casing, plus 2 for the dust ruffle, plus three for the hem. The valance to be full enough will likely need two cuts of equal length for each window. Most would center one cut, and split the other and add the width to each side.

Thanks gals for your response! I guess (after re-reading my post)..I wasn't very clear on the top hem of both the drapery panels and valances.
I have the double regular curtain rods (white) at each window. I had read that I needed to allow 8" for a top & bottom hem-but I don't need that much. I only need a curtain rod pocket of 2", and a top 'dust ruffle' of 1"...as I will be using shower curtain hooks (with the stailess roller balls) on them on the thin rod-so they'll slide easily to close. Also they'll be hidden under the valance.. The bottom of the panels only need to be a 3" hem. I'll need a 'dust ruffle' on the valances of 2" and the valance hems only need to be 1". Sorry if I was clear as mud. Thanks again!

I am retired from selling and installing draperies. Figure your fullness at 2.5 to 1 or 3 to 1. They will look much better than 2 to 1 fullness. You lose some fullness when you sew in side hems and seams. Length + 14" is OK for cuts.