This is how I cutt bottles. I built this jig to make a clean and straight scar perpendicular to the bottle's vertical axis. The threaded rod and nuts allow the glass cutter to be adjusted to cut different bottle lengths. After marking the scar, I pour boiling water on it and then I plunge the bottle into icy water. Sudden temperature change makes the bottle to split in two leaving a perfect cut. Then I smooth the glass edges with fine grit sand paper.