You could divide it in half (like you said) and use it as a top and bottom border. If you don't have quite enough, you could center it and add part of the sheet to make it long enough, then use the sheet as a side border as well. Or you could use the body of the sheet as a backing. Do you have a pattern in mind for the top? I think this is going to be beautiful, no matter what you do! What a wonderful way to remember your Grandma! :D