At my airline (European legacy) we only have dispatch landing tables for a flaps 30 landing. We add additional fuel if landing with flaps 20, as required by the low fuel qty procedure, would cause us to exceed the available ldg distance according to the Performance Inflight tables.
Maybe the reasoning is that the use of contingency fuel occurs only after dispatch.
We have tons of lawyers and guys with slide rules in service, so I can only assume that this method has the blessing of the regulators.