HI TF, As soon as you pass the 100% profit mark in K's way of calculating it, then your average cost per share is less than zero.
This brings to the point why this is improper calculating for tax purposes! We certainly don't want to pay tax on a capital gain showing a negative cost per share!
The calculation that I presented to Lou in another post shows the proper ave. cost calculation for doing one's taxes on an average cost basis. It would be impossible for the average cost to drop to zero or below in that way of doing the calculations.