I am pretty sure you cannot use average price across multiple parcels of shares when selling all or part of an individual parcel. You have to deal with each parcel or part parcel separately both in quantity and price. This may mean that a single sale may need to be split across several of the parcels purchased and the profit/loss on each calculated separately.
If you choose to sell say 5,000 shares from the 5/3/15 parcel then profit/loss will be calculated on the $7 paid and you will have a parcel of 600 shares @ $7 purchased on 5/3/15 remaining. If you later sell another 2,000 shares you may choose to split this into the 600 remaining from the 5/3/15 parcel and 1,400 from one of the other two purchases. The profit/loss on each (the 600 and 1,400 parcels) will need to be calculated separately. The calculation will also separately account for the cgt discount on each parcel if applicable based on the date of purchase of each parcel.
