Pinkiris, I marked my quilt because I just didn't trust myself to sew straight enough over this size. If you do, then just go with marking the center diagonal and just sew. On a smaller one I might not mark all the way.
Quilter in the making: yes a half inch all the way across. Still the fastest baby quilt I've ever made! I think I'll try three layers, slash 2, and sew every quarter inch to see what that looks like next!