So you need to "match up" shares. Let's say you bought 5k shares, then 10k shares, and later 5k shares, etc. You sell 20k. So the 20k shares you sold get broken up into 5 + 10 + 5k. You divide the sale revenue from the 20k into 25% + 50% + 25%. Each share purchase then has its own tax calculation done on it.
In the case where your share numbers don't match, so you bought 10k shares, but are selling 5k, you divide that original purchase into 2 x 5k lots. Then you can match 1 lot against the 5k sale. The other 5k buy remains to be matched at a future date.
