also, here is the link to the OBW Design Helper, where you can upload a photograph of your fabric, and see how it will look made into hexies of various sizes http://oneblockwonder.com/design-helper/

use the lengthwise if you have enough! Unfold the bolt crease so the fabric is just one layer. Then fold it end to end and align the selvedge. fold it again and the length should be under 24" so it fits on your mat and your 24" ruler. As long as you start with the selveges aligned and your first cut is exactly parallel to the selvege your strips should be perfectly straight.