What the calculation measures
A simple average of purchase prices is wrong when lot sizes differ. Weight each price by the number of shares in that purchase.
How to interpret the result
Use lot-level broker records for tax reporting and wash-sale adjustments.
Run scenarios instead of one answer
Change the most uncertain input and compare outcomes. A calculator is most useful when it shows sensitivity, not when one estimate is treated as a guarantee.
Limits and assumptions
The calculator is a planning aid, not an official tax basis statement.