With the tank mounted on your BC at the proper height on the plate, the regulator you will use with the port you choose for the hose on the valve, with the BC on and adjusted to fit, standing up or on your knees so someone can reach the valve. Connect the hose to the BC, loosely run the hose from there to the port, the hose should not bell out away from your body and it should not pull straight from the port. At that point have the other person slap a piece of tape on the hose where the threads would be, if the hose were cut. Since you are hanging the tank from your shoulders, it will be about 2 inches longer than it would be if you were neutral in the water. I would not worry about the extra 2 inches. This is a judgment call and one of the best ways to determine hose length. Any other answer is going to get the "usual BC hoses are" - the "most BC hoses are" - "the wing manufacturer recommends". There are many variables in what you can setup "single tank" that will change that length, so be sure you are setup properly with the wing on the plate for the tank you are going to use and the plate is where it needs to ride on your body. Doubles or "twins" are another brother.