Well, Han, SPO.POL.140 requires you to make allowance for "expected variations in the approach and landing techniques" in regards to the POH data (which is flight test data, usually derived under perfect conditions). And in reference to AMC2 SPO.POL.140 you need to state this allowance in the OM.There is a similar statement for take-off in SPO.POL.130, which requires you to clear obstacles by a "adequate margin". These margins are not described in Part-SPO, but reference is made to ICAO Annex 6 Part I. And if you look there, you'll find the same margins as described in Part-CAT. So all in all you have more flexibility in Part-SPO, but you still need to define your margin (which obviously cannot be 0).

The in-fligth determination has actually a bit less margin required, if you have actual performance information (i.e. weather and RCC), if you have made a proper allowance in your pre-dispatch planning. Let me give you an example. In our operation we use a landing distance factor of 80% (which equals a factor of 1,25).
Now, if the LD is 1000m, the LDR therefore is 1250m (pre-dispatch). In-flight the LD would still be 1000m (if no values like mass have changed), but if we have landing performance information, we only need to apply 115%, which gives us 1150m. If we do not have performance information, the LDR would remain at 1250m.

Hope that helps ;-)

Best regards,

Markus