Always identify the base
A percent calculation depends on the original base value. Ten percent of 200 is not the same as ten percent of 180, and a discount followed by tax is different from tax followed by discount.
When reading a sale label, ask what number the percent applies to. Retail discounts usually apply to the original price, while fees or taxes may apply to the discounted subtotal.
Markup and margin are not the same
A 25 percent markup means the increase is 25 percent of cost. A 25 percent margin means profit is 25 percent of the sale price. These formulas produce different results.
For business estimates, write the formula beside the result so another person can tell whether you used markup, margin, or a simple percent increase.
Round at the end
Rounding each step can create small differences. For receipts, invoices, and shared bills, keep intermediate values precise and round only the final amount to cents.
When splitting bills, confirm whether the tip is calculated before or after tax, because local habits and restaurant systems vary.