Haroon, making simple things complicated won't make your life easier. Airbus has introduced operational landing distances containing necessary margins and covering various runway conditions, aircraft status and failures. Make it simple: Say you're flying a long haul for 10 hours after 5 hours upon entering some FIR ATC tells you to divert to the airdrome X, with LDA 9000 ft, due to airspace being closed for whatever reasons. Now your actual landing distance with current GW, as per QRH in the flight performance table, is 7000 ft. If you apply it you can land safely if you think to re-plan then you'd need 11690 ft. No brainer there, if you ask me.
