Simplify calculation

lib/process_metrics.js

@@ -41,8 +41,7 @@ var process_metrics = function (metrics, flushInterval, ts, flushCallback) {
         for (key2 in pctThreshold) {
           var pct = pctThreshold[key2];
           if (count > 1) {
-            var thresholdIndex = Math.round(((100 - pct) / 100) * count);
-            var numInThreshold = count - thresholdIndex;
+            var numInThreshold = Math.round(pct / 100 * count);
 
             maxAtThreshold = values[numInThreshold - 1];
             sum = cumulativeValues[numInThreshold - 1];