Estimate candle burn time

How do I estimate Candle Burn Time?
1.  Weigh your original candle before burning.  (eg 8oz)  

2.  Then burn for 3 hours.

3.  Weigh it again after you blow it out.  (eg. 7.4oz)  

4.  Take the difference in weight (eg. 0.6oz)  

5.  Divide by 3 (for the # of hours burned) (eg. 0.2oz/hour)  

6.  Divide original weight by rate.  (eg.  8.0oz/0.2oz = 40hours)