Found my online stats calc code. It's in C but I can rework it. This should work better for FPS calc.

stats.c

@@ -0,0 +1,65 @@
+#include <math.h>
+#include <lcthw/stats.h>
+#include <stdlib.h>
+#include <lcthw/dbg.h>
+
+Stats *Stats_recreate(double sum, double sumsq, unsigned long n,
+        double min, double max)
+{
+    Stats *st = malloc(sizeof(Stats));
+    check_mem(st);
+
+    st->sum = sum;
+    st->sumsq = sumsq;
+    st->n = n;
+    st->min = min;
+    st->max = max;
+
+    return st;
+
+error:
+    return NULL;
+}
+
+Stats *Stats_create()
+{
+    return Stats_recreate(0.0, 0.0, 0L, 0.0, 0.0);
+}
+
+double Stats_mean(Stats * st)
+{
+    return st->sum / st->n;
+}
+
+double Stats_stddev(Stats * st)
+{
+    return sqrt((st->sumsq - (st->sum * st->sum / st->n)) /
+            (st->n - 1));
+}
+
+void Stats_sample(Stats * st, double s)
+{
+    st->sum += s;
+    st->sumsq += s * s;
+
+    if (st->n == 0) {
+        st->min = s;
+        st->max = s;
+    } else {
+        if (st->min > s)
+            st->min = s;
+        if (st->max < s)
+            st->max = s;
+    }
+
+    st->n += 1;
+}
+
+void Stats_dump(Stats * st)
+{
+    fprintf(stderr,
+            "sum: %f, sumsq: %f, n: %ld, "
+            "min: %f, max: %f, mean: %f, stddev: %f",
+            st->sum, st->sumsq, st->n, st->min, st->max, Stats_mean(st),
+            Stats_stddev(st));
+}

stats.h

@@ -0,0 +1,25 @@
+#ifndef lcthw_stats_h
+#define lcthw_stats_h
+
+typedef struct Stats {
+    double sum;
+    double sumsq;
+    unsigned long n;
+    double min;
+    double max;
+} Stats;
+
+Stats *Stats_recreate(double sum, double sumsq, unsigned long n,
+        double min, double max);
+
+Stats *Stats_create();
+
+double Stats_mean(Stats * st);
+
+double Stats_stddev(Stats * st);
+
+void Stats_sample(Stats * st, double s);
+
+void Stats_dump(Stats * st);
+
+#endif