Using the straight edge and possibly cutting the bottom layer is definitely high risk. That was a one and done improvement onlyI’ve got that same 12’ straight edge but I don’t use it for cutting carpet on top of carpet, such as you are by cutting on the roll. I’ve accidentally scored the layer underneath by doing it that way so now I put a tick on each side and chalk line my cut. You can lift up the carpet off of the next layer of carpet and cut it while still following a straight line.