Put in one menu item. See your food cost %, the margin it leaves per plate, the price that hits your target, and what a small price change is worth.
For a single dish, many cafés work to around 28 to 32% as a working target, not an ATO figure. Across the whole café the ATO range is higher (34 to 42% under $250k turnover, 35 to 41% from $250k to $600k, 33 to 38% over $600k) because it includes waste, staff meals and drinks.
Divide the menu price by 1.1 to take out GST, then compare that with your food cost. The GST is not your money. Use ingredient costs without any GST you can claim back.
Whenever a key ingredient price changes, and at every menu change. Keep portions and recipe cards up to date.