Think about laying 2 pieces of your 45" and sewn together (selvages to selvage). This would make a piece approximately 84" wide (remember fabric is no longer 45" and you have your selvages). This gives you your seam horizontally. Now, you need the width - 56". You need 2 pieces because you used 2 lengths, so 2 x's 56" = 112". 108" is 3 yards, so you could get by with 3 1/8 yards. I would probably buy 3 1/2 yards. Hope this helps. If you aren't good at drawing things out on graph paper, you might take an old bed sheet , lay it out and with a sharpie, mark sizes on it (45", 54" 72", etc). Then use can lay it on the floor, fold it back to your expected quilt top size and lay out fabric on top of it to see how much you need. This is a bit more cumbersome, but, if you have a difficulty with paper drawings, this works!

If the amount of fabric is "not quite enough" - one can always put a contrasting band/strip - or band/strips - to make it look like it was a design choice.

I had - at one time - assumed that all quilting type fabrics were 45 inches wide. I had this top that was about 84 inches wide - should have been plenty. WRONG! That's when I learned about adding strips to make it look like this was what I had really wanted to do all the time anyway!