I just checked my JT slotted ruler. It's perfect at 4" with every ruler I have except the 6" square Omnigrid. My others are Simplicity, Omnigrid and June Tailor. What ruler am I using on this quilt to square up blocks cut with the slotted ruler? You guessed it - the 6" Omnigrid. Oh, well. As long I stay consistant with my square up ruler, I'm okay.