When you are cutting your six strip set:
Your square ruler should be bigger than your strip set.
Cut #1: position the 45* diagonal line on your ruler on the seam line, trim off the end.
#2-Every cut after that: position the diagonal line of your ruler on the (longer on the fabric tube) seam line, slide ruler until the edge of your ruler slides off the fabric on the at the opposite (short on the fabric tube) seamline. the ruler should be right AT the short seam, but not on it. Check both positions, then cut from the short fabric side to the long fabric side. The side where the 45* diagonal ruler line is, the seam allowance is hanging off past the ruler corner, that's okay, the ruler will guide you to cut it. Open this piece up. It should be a perfect square.
Without pivoting the ruler, slide the 45* diagonal line to the opposite side, repeating from #2.