Tipping norms vary
Around 15-20% is customary in the US, ~10% in much of Europe when service isn't included, and tipping is minimal or already included in the bill across much of Asia. Check whether a service charge is already on the receipt before adding more.
Quick mental math
10% is just the bill with the decimal shifted left; 20% is that doubled; 15% is 10% plus half of it again.