Olendirk,
At the Pre-Flight phase, you must ensure that the Landing Distance Available (LDA) is equal to or greater than the Demonstrated Landing Distance, factored by 1.67 for DRY runways, or 1.92 (1.67 X 1.15) for WET runways.
If you dispatch with a known MEL affecting landing distance, then the extra factor (or distance) for the deviation must also be factored. For example, Anti-Skid Inop may specify that the Landing Distance Required for a dry runway is 1.3 times that for normal. In this case, the LDA must equal or exceed LDR X 1.3 X 1.67, i.e. LDR X 2.17 at the time of departure. If however, the aircraft departed with no abnormalities, but Anti-Skid became inoperative in flight, you may then use the 0.67 'buffer' provided for at the time of departure. Put another way, you dispatched with 0.67 X LDR available as a buffer, and you may now use some of it, i.e. 0.3 X LDR to accomodate the deficiency.
Regards,
Old Smokey