I figure how many pieces I can get across the width of the fabric (WOF) (ex. - need 4.5" squares = divide useable WOF -measurement minus selvages - by the 4.5 to find how many squares you can get from one WOF strip), then divide the number of squares I need by that number to get the number of strips to cut. In my example, having 41" useable in the WOF I can get 9 4 1/2" squares in each strip. If I need 60 squares of that particular fabric, divide 60 by 9, so I would need to cut 7 strips to get the needed amount. Hope this is clear, I think we all figure a little differently. A suggestion - could you put your cutting mat on your ironing board if you need to cut an additional amount?