RigChalk is the handy quick-calculator to figure out bridle math when it come time to actually chalk the floor. It's efficient and will figure out the steel you need to draw the point and provides only points within the specs you choose, including looking out for points that may be too tight to be safe. Works with metric and imperial steel & venue information, and lets you choose from the supplies you have on hand to provide you with buildable, hangable bridles.
It almost goes without saying that RigChalk should always be used purely as a tool to assist professional riggers and any information it provides should always be approved by a lead rigger before it's used. Safety is #1 in rigging and should always be taken very seriously.