hopetoquilt

I have three quilts going to a LAQ. The first two are 70 inches square. I will post pix below. The third one will be a white and dark blue storm at sea and will be 75 inches square. I have the pieces cut and the specialty ruler so I expect to throw it together quickly. I need to order backing fabric and I want to use wide backs. I love batiks and some tone on tone patterns. I would like decent quality fabric without paying an arm and a leg. I just dumped about 5 potential fabrics from thousands of bolts into my cart and they don't have enough fabric for any choice. Also want to double check... How much extra do I add total to the length for a quilt. I know a wide back is clearly wide enough.

i am looking for good on line stores to shop with decent prices and some suggestions on colors. Can I do one fabric for all three. The two that are pictured... I think I could do purple. Looking for some advice.