Keeping in mind that there is a major difference between share...

Keeping in mind that there is a major difference between share investing and share trading.

If you make your money by buying and holding shares, then you are an investor.
If you make your money by frequently buying and selling shares, then you are a trader.
The line between the two is murky. Talk to your accountant.

The big difference between the two is how tax is calculated. If you are an investor, then capital gains tax laws apply. They're complicated, so talk to your accountant. But basically you pay tax only when you (eventually) sell the stock. And you need to decide which stock you've sold.

If you are a trader, the calculations are a lot simpler. Let's say you're calculating your gross income for the 18-19 financial year:

= Value of stock as of 5pm 30-Jun-18 (Previous stocktake)
+ Total sell price of all of the stock you've sold this fiscal year (Income)
- Total buy price of all of the stock you've bought this fiscal year (Deductions)
- All other deductions, such as brokerage fees, analyst subscriptions, etc.
+ Value of stock as of 5pm 30-Jun-19 (Current stocktake).

I prefer to be a trader since I don't need to decide which stock I've sold. It's all calculated based on value. In your example:

Buy 15000 @ $4 and 15000 @ $1.60
= $84,000 deduction

Sell 15000 @ $2.10
= $31,000 income

Currently you have 15000 shares each worth $2.10
= $31,000 stocktake

31000 (income) - 84000 (deduction) + 31000 (stocktake) = $22,000 loss.
Therefore your taxable income would reduce by $22,000.
There is no need to work out your average value, or decide which stock you've sold, if you're a trader.