Pre Dispatch the factor is applied with forecast landing weight but no credit given for reversers and planned using max manual brakes... Realistically this configuration is never actually used, hence the factor to achieve operational factors.
Actual inflight distances take into account actual landing weight, actual weather and runways conditions, actual configuration used such as auto brakes instead of max manual braking and use of reverse thrust vs no reverse or idle.
If unable pre-dispatch to use destination due forecast weather conditions, two alternates are required. If during the flight conditions at destination improved a new 'dispatch' calculation can be made which may well allow landing at destination, but rather than making two calculations only inflight calculations are now required.
The 305m air distance comes from the FAA 1000' markers used. ICAO national generally have markers set at 400m, and this correction of an additional 95m should always be taken into account. Actual location of the markers can be found in the AIP for the airport/nation thus when performance is limited the actual value should be used to ensure actual distance available for calculation purposes.