There is no international figure
The percentage printed in a guidebook is an average of a country that does not have an average. A restaurant in the same city will differ from a taxi by more than either differs from its neighbour country. A house that travels with one number in its head will be wrong somewhere, and will usually be wrong in the direction of over-tipping, which is its own small awkwardness.
What travels is the method: ask. A concierge asked on arrival, in one question, will tell you what the house needs to know for the week, and will be right in a way no published table is. The question is not an admission of ignorance; it is what a well-travelled guest asks first.
Where it is expected
In the United States a gratuity is part of a wage and the figures are real: fifteen to twenty per cent in a restaurant, a note per bag for a porter, a note per round in a bar. In Canada much the same. In much of the Gulf a service charge is already included and a small addition is a courtesy rather than an expectation.
In the United Kingdom and Ireland a discretionary service charge of twelve and a half per cent is common and can be removed, though a house that removes it should have a reason it is willing to give aloud. In France and Italy service is included by law and a small rounding is the whole of the answer.
Where it is not
In Japan a tip is often refused, and pressing it can cause real discomfort; the courtesy is the way the money is handed over at the counter, in full, without counting change. In parts of Scandinavia and Switzerland service is included and a rounding is sufficient. In Singapore tipping is generally not practised in restaurants and is not expected in taxis.
A guest who tips where it is not the practice has not been generous; they have made a person decide whether to refuse money in front of them. Where the house is unsure, it asks, and where it cannot ask it follows the room.
How it is given
In local currency, in notes, handed over rather than left, and with the person named if the name is known. A note left under a glass is acceptable in a restaurant and less so in a house where staff were introduced. Where several people looked after the table, the gratuity is given to the one who ran it and the house says so plainly.
Never counted out coin by coin in front of the person it is for. A gratuity is not a verdict on service; it is the ordinary cost of being looked after by people who are paid to do it and often paid little.
What a household writes down
A house that travels keeps its own short record, country by country: what was usual, who was generous, what was refused. That record is worth more to a family than any page on the internet, because it is the practice that household actually met, with the names of the people who explained it.