I'd cut the back generous as quilting sometimes will effect how all ends up. You can then adjust the backing when you are ready for the final step, Use the same measuring device throughout these steps. My yardstick is a metal one, not wood. It is thinner than a wood one and I think the measuring might be more accurate? I haven't tried a measuring tape but those are flexible where the metal yardstick is flat and sturdy.

I bought a metal yardstick years ago just for the purpose of measuring backings. I got it at Lowe's.

Audrey, hi and welcome to the board and the wonderful world of quilting? You will always get responses to your questions here and alternate methods of accomplishing what your trying to do. Personally I use the kind of measuring tape you would use on wood the metal retractable kind just seems to work best for me. Good luck with your project and be sure to post a picture of your completed quilt.

You are going to cut your backing bigger than your top, so don't worry too much. If your space (and experience) are limited, try folding the piece in quarters and then measuring along the 2 fold lines. This measurement will be half what you need (example quilt top is 60" wide, you will want to cut a piece 68" wide, so measure 34" from fold, mark and cut. When opened, it will be 68". Then do same for length). As you get more experience, you will learn easier ways to lay out fabric and cut it. Just remember, there are very few hard rules, it is what works best for you.