Thats the stratedgy I use aswell, however, I usually make the sample time period longer. Anyone can do something 100% for 5 min, but what about an hour? I generally use atleast 10 min, and more commonly 30 min, to figure out my hourly time. That way, you wont be dissappointed if you can't keep the pace up. For example, when I was powermining iron, I could get 7k in ten minutes, which should be 42k an hour, however, I could only get an average of 40k per hour. I suppose it isn't that big of deal, but it does make your calculation more accurate, and its always better to underestimate, than overestimate.