Following on from the previous post,
(a) the actual minimum landing distance is something the test pilots achieved. This was after some dedicated practice work and, even then, the "bad" test runs will be discarded by the OEM TPs and a few more by the backroom aerodynamicists analysing the test results data. The OEM/Regulator TPs will come to agreement on what figures they are going to OK for the aerodynamicists to use in developing the AFM (certification) data.
Having been in aircraft during maximum performance landing tests, might I suggest that you and I, as mere mortal line pilots, are NOT going to achieve what the TPs have put in the AFM as actual figures. That is to say, we will end up in the fudge factor region to a greater or lesser extent, even if we reckon we have done a real beaut job of getting the aircraft on the ground and so forth.
(b) the operationally required landing distance includes fudge factors to provide some fat. Typically, these are 1.67 for dry and 1.67 x 1.15 = 1.92 for wet conditions. Note, if you wish, 1/0.6 = 1.67.
Regulation states: JET have to stop within 60% of the LDA.
Perhaps, providing you read LDR instead of LDA, but let's not read that literally. What it means is that the required AFM landing distance x 0.6 will approximate the flight test actual landing distance. You and I are NOT going to achieve that, as a general rule, regardless of how well we do the task. The statement should be read as "you need to have a 1.67 factor on top of the flight test actual distance achieved for the AFM required landing distance".
Providing the runway LDA is longer than the factored AFM LDR figure then you should be reasonably safe to operate. Do keep in mind, though, runway surface friction characteristics need to be taken into account when considering the landing exercise.
Some jurisdictions permit 1.42 (in lieu of 1.67) for heavy props and alternates but the basic idea remains the same.