How about the ruler method of folding fabric. Because you have such deep shelves, after folding your fabric around the 6 x 24 ruler, slip the fabric off, and just fold it once, in half. Your fabric will then measure approx. 6 x 22 (width of ruler x ½ the 44 width of fabric)
Here is a tutorial to help you,
http://www.youtube.com/watch?v=rkTlaMqRlwo