This topic came up when I was trading and the answer is that there are no fixed rules such as "first in, first out". If selling some shares you have accumulated over time you can chose any parcel, or part of, to be the purchase price of the parcel sold.
If you are selling at a profit you choose those you've held over 12 months for eg. If selling at a loss you choose those that best fit your circumstances at the time.
Hope this helps but I am unqualified to give financial advice.
