Is it not the case that use of reverse thrust is not factored into Landing Distance required? (Rhetorical question, it is not.)
Therefore that aircraft will stop as per the "book" regardless of reverse use, be it full, idle or none at all.
Boeing, at least, uses an autobrake logic to stop the aircraft using a fixed level of deceleration, and all reverse does is to relieve some of the load on the brakes. It has no effect whatsoever on landing distance.
It may be that other manufacturers use a different logic, but in a Boeing (probably 70% of traffic at STN) use of reverse will not affect landing distance - unless, of course, the tyre adhesion can't cope, as in contaminated conditions, when it could make a significant difference.
Why, then, use reverse at all, you may ask. Simple. $$$
Less work for the brakes means brakesets last longer, and brakesets cost big money. Idle reverse makes no more noise than no reverse which is why many airlines use idle only - saves dosh, saves wear and tear with no noise penalty, = win win for a change!
Ps. To all those who say in a pre-landing brief, "runway is short so I'll use 70% reverse" think again, just as "runway is wet/slippery so I'll use autobrake 3". Think about it. Just ain't logical!