There are too many variables (payload, cost index, cruise speed, winds, temperature) to calculate a single value. The good news is that you have a pretty powerful computer at your fingertips -- the B757's FMS. Try some routes for yourself, with whatever payload and fuel load you want. You can also input whatever fuel reserves you want to keep (e.g. 20%). If a particular route can't be flown with your payload, fuel load, or reserves, then the FMS will give you an INSUFFICIENT FUEL warning when you program it. For more detail, you can check the LEGS pages to see what the predicted fuel is as you pass each waypoint and where the fuel would run out or pass below your reserve level.