Calculate tip amount and split the bill evenly among any number of people.
The calculation is a percentage of the bill, then a division if the bill is being split. The two decisions that actually change the number are whether you tip on the pre-tax or post-tax total, and how you handle rounding. Tipping on the pre-tax amount is the more common convention where a service charge is not already included, since tax is not part of the service being rewarded, though many people simply use the final total for convenience.
The larger point is that tipping norms are cultural rather than mathematical, and the same percentage means very different things in different places. In much of the United States tipping is a substantial part of server income and 15 to 20 per cent is customary; across much of Europe a service charge is frequently included and rounding up is sufficient; in Japan tipping can cause genuine awkwardness and is generally not expected. In India, many restaurants add a service charge to the bill — and it is worth knowing that this charge is not legally mandatory and can be declined, which is separate from whether you choose to tip.
Pre-tax is the more logical convention, since tax is not part of the service. Many people use the final total for simplicity, and the difference is usually small, so either is defensible unless a service charge is already included.
Then a further tip is generally not expected, though a small additional amount for exceptional service is a personal choice. Check the bill before adding, since paying twice for service is a common mistake.
Apply the tip percentage to each person’s own share rather than splitting the total tip equally. That way each person tips in proportion to what they ordered, which is fairer when orders differ substantially.
No, and assuming it is can cause offence. Norms vary widely by country: it is substantial in the United States, modest or built in across much of Europe, and generally not practised in Japan. It is worth checking local custom when travelling.