Thanks for your input Oyvoyvoyv (difficult name to type, that).
Also, I will take the opportunity to point out that my meaning is only to track changes, not predicting when or how much, hence my "Estimated weeks to 50% marketshare:" is simply the Gap divided by the gap difference :) As more data flows in I might do revised estimates, but pretty much any estimate is going to be bonkers because of christmas coming up.
Still, I pretty much agree with your estimate there.
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