The formula
The tax inside the receipt is gross × rate ÷ (1 + rate). The operator’s percentage comes off that, then any fixed fee per transaction. If the receipt is under the minimum spend, the refund is zero. A refund is never negative.
With the UAE figures quoted by the Federal Tax Authority, a 1,050 receipt contains 50.00 of VAT; 87% of that is 43.50; minus AED 3.60 leaves 39.90, or 3.8% of the receipt. See the UAE refund breakdown.
What it assumes
The estimate assumes you are eligible, the store takes part, and the goods are exported in your luggage and validated in time. Goods shipped home by freight do not get a tourist refund. Read how tourist VAT refunds work before relying on one, and use the landed-cost calculator to add card fees and import tax at home.