The answers given so far are on the right track. If you are using a glass cutter wheel, you need to build a balance apparatus to ensure that the scoreline is completely even. The method of heating and cooling is the scoreline is probably the safest--tie a piece of metho-soaked cloth around the scoreline and set it alight, then run a fresh icecube around it immediately the flame falls off. The more traditional method of gently tapping the glass on the inside of the scoreline will not give you the straightest/safest cut. In any case you will probably have to put protective stripping over the cut, or take it to a glazier for grinding--in which case, I would let the glazier grind the whole thing in the first place--his risk, not yours. PS: Why not experiment with a plastic one first? You may be saving some land-fill.