Sorry mate. Can't find any reference to weight margin in the Airbus FCOM's. The limitations are 1. Max weight 2. 2nd segment 3. Runway length - one engine out 4. Obstacles 5. Tyre speed 6. Brake Energy 7. Take-off dist - all engines 8 Final take-off.
Those codes are straight out of the FCOM.
Can't say much more than that. Only to repeat that 3-4 would be a one engine out limit followed by a less limiting obstacle weight.
Off to Airbus in Toulouse for a course. Hope it helps