Unless the rules have changed without me noticing, there is no...

Unless the rules have changed without me noticing, there is no need to use the FIFO method if another way gives you a better result. What is important is that you clearly document which parcel or parcels the shares came from. They can all be from one parcel (first, last or any in between) or from multiple parcels. If from multiple parcels, you will have to do separate calculations for each because the purchase prices and dates will be different; you cannot use an average purchase price. Do confirm this via a phone call to the ATO though.