Using anything other than the immediate hash-rate would cause the clients pools to weigh in on the current speed.  Every test shows that immediate hash-rate causes the least false positives and the most accurate results.  As for +/- 5% it sounds like a lot of providers rate their rigs at -1% to -5% in order to deliver the promised hash-rate even on slow pools.  Keep posting, we'll follow the feedback and see where it leads.