So my simple calculation was more accurate. Incredible!
I still don't understand why you use so weird math, and use weeks instead of days. Average sales/earnings per 15 days gives pretty accurate results. Median is also good. And of course the glance look to previous year.
But weeks... I think they are more chaotic, like days. Sometimes you have good days, sometime bad. The same with the weeks. It's more about probability theory.
UPD: And when I have bad day or whole week with no reason why. I think not about "something wrong with sales", "we all die", but about probability... "I have bad days, so very soon I will get good ones".