final SortedMap<SourceLevelKey, TargetLevelKey> tailMap = sourceToTarget.tailMap(sourceKey);
if (headMap.isEmpty()) {
final SourceLevelKey firstKey = sourceToTarget.firstKey();
final TargetLevelKey firstValue = sourceToTarget.get(firstKey);
final Level firstLevel = firstValue.getLevel();
return registerMapping(firstLevel.toInt() + 10000, firstLevel.getSyslogEquivalent(), sourceKey);
} else if (tailMap.isEmpty()) {
final SourceLevelKey lastKey = sourceToTarget.lastKey();
final TargetLevelKey lastValue = sourceToTarget.get(lastKey);
final Level lastLevel = lastValue.getLevel();
return registerMapping(lastLevel.toInt() / 2, lastLevel.getSyslogEquivalent(), sourceKey);