I wouldn't trim anything yet. Use a yardstick to line the bottom points of your hexies with the lines on the backing, if you can see the lighter side of the stripes. Then I would pin baste the outsides of your top and batting. It won't be perfect but can be close. If there is a better idea, I'm all for it. You don't state how you basted it. Most I've seen and I have done myself, I baste all 3 at once. batting to backing then top to batting if spray basting. If the stripes are thin it is little more difficult. let's see what everyone else says.