To get the length you need add the extra fabric around the edge of your top, usually 4" for hand/domestic quilting. 6-8" LA.
DSM--- 115+4=119. Width: Divide 119/40=~3 (you will loose almost 2 inches in selvage/seams) for # repeats of fabric. Length: divide 119/36 = 3.33 or 3 1/3. Next multiply length x width. 3*3 1/3= 10 yards.
LA---115+8=123. W: 123/40=~3.02. L:123/36=~3.42. WxL=3.02x3.42= 10.33, I would round to 10 1/2 yds.