I need a correction on the profit and Loss Report

(Sales Revenue- Sales Tax) - Cost of Goods Sold = Gross Profit
Gross Profit - Expenses = Net Profit

The cost of goods is derived from the cost attached to what has been sold and it should be average cost i.e

when i sell 3 items of x each costing 20, 30 and 40 the average cost is 20+30+40 /3 =30

so my profit and loss will look like this

sales ...................... ..................... 77.57 ex vat price
Less Cost of goods Sold .................... 25.86 ex vat average cost
Gross Profit ......................................................................... 51.86
Less Expenses rent 10
Net Profit 41.86

is possible let this be inform of a report on grid

n/b if no tax is activated the figures remain as they are in whole without removing tax

i would also like to confirm that this is one of the best systems based on design and process.
Kindly advise