For planning purposes the charts show a 300 tonne airplane requiring about 8000 feet for dispatch to a sea level airport with no wind and normal brakes on a dry runway and about 9200 feet on a wet runway. So the plane should be able to come to a stop in about 4800 on a dry runway (60% of 8000). The extra 1200 is the additional 15% to release the aircraft to a wet runway.
The unfactored advisory landing distances get interesting. There are tables for a dry runway, good braking action, medium braking action and poor braking action. For a 250 tonne aircraft both the dry chart and the good braking action chart show the aircraft stopping in 6420 feet with auto brakes 3 and flaps 25. In both cases you must add 80 feet if only two reversers are working and 110 feet with no reverse. It would take 6770 to stop on a medium braking action runway and you must add 460 feet for two reversers and 1810 for no reverse. And on a runway with poor braking action it looks like you'll need 7860 to stop and you must add 1550 for two reversers and 5040 for no reverse.
Working the advisory distances at 300 tonnes it looks like with max autobrakes the aircraft should stop in 4600 feet on a dry runway which validates the dispatch numbers. On a wet runway with good braking action it should take 4950 to come to a stop. But on a runway with poor braking action a 300 tonne aircraft will need at least 8000 feet to come to a stop.
Hope that helps.