Interesting scenario.
Airbus, or indeed any manufacturer, can not provide procedures and calculations for every possible combination of failures. So when something like this happens, you are are both test pilots.
We have an aircraft in a severely degraded state. The configuration is so abnormal that it was not considered by the manufacturer.
Why attempt to calculate the required LDR distance at all? Any figures you come up with will be at best just educated guesses.
Locate all the airports you could reasonably get to with the fuel onboard, considering of course the fact that you will have to fly there with gear down and F3.
Pick the runway that is going to slow down the aircraft the most by the time you get to the end of it.
And if you go off the end of the runway?
Well, it would be pretty hard to argue against your decision from either an airmanship, or legal, point of view.