I just received the new QRH landing distance calculations. The way I see it, there is a column at each runway condition table which has the corrections for reverse usage.
For me, this Airbus SOP line means that once I have calculated the landing distance required, and the runway is long enough to stop even with idle reverse, I can legally use idle reverse.
Ps: I hate the term "legally". To me, it says that I am not concerned and regulated by the rules of physics anymore, but by some legal mambo-jumbo...
And for those who say we should always use full reverse as a safety precaution, I suggest to always fill up all the tanks with fuel and refuse half of the passengers, as this way, the aircraft will have plenty of extra fuel, but still light as payload does not degrade the landing performance... As for braking, always use max manual as this way, the aircraft will always stop on the runway. And if there is just chance of even a slight drizzle, don't even leave...