You don't have to pull out the stock wires, just get a measuring tape like they use for tailoring clothes. Run it loosely where you want to run the wire and add up the measurements. I'd by an extra foot or so to give you room to play with...
If you don't have a measuring tape, you can always use some string or, preferably stiff rope (simulates the wire's inflexibility). Then either measure it with a ruler or simply take it to a store and have an employee cut the same size wire...