I wouldn't trust a number unless I could verify it or make a reasonable estimate and add some extra.
One method that may work is to remove the hose at the tank and use a snake (like an electricians snake) inserted into the fill fitting. Push it through until it comes out the open hose in the tank coffin. Mark the snake at the fill and pull it out. Now measure the length of the tape and you will have an accurate length. The vent hose is a little harder if you have a combo fill/vent gas fitting. If you can get to the underside of the fill, remove the vent line at both ends and use the snake as above. I would not be surprised if the vent hose is longer than the fill hose. It may be if it has a high mounted loop.