I’m not a expert at all but I do prefer to cut my borders LOF (length of fabric) vs WOF (width of fabric selvage to selvage). I would go ahead and cut your 6.5 in wide strips (WOF) and join strips to get the length needed. 6.5 x 4 =26 inches total width, I think you should be able to get 4 15 inch strips from cutting one additional 6.5 in strip. Adding a 15 inch strip to each 60 inch strip should give you what you need, unless your mitering your border. I’m sure I’m clear as mud and I’m sure someone else will be along to better answer your question. We’ve all been in similar situations many times!