There is no endurance calculation based on leak rate because there is no guarantee it will remain same. The first action in QRH after leak is detected is Land ASAP. So you need to know the range more than endurance. The situation will be either the leak stopped but one engine shut down or leak continues from wing or centre tank with both engines on and both of these will affect range/endurance. Safer diversion strategy is to consider that tank fuel lost and airfield within that range should be chosen.