Calculate the tip and split the bill between friends.
Tipping norms vary enormously by country, and the calculator simply applies whatever percentage you choose. The harder question is what percentage is appropriate.
In the United States, restaurant servers are frequently paid a reduced base wage on the assumption that tips make up the difference, so 18 to 20 percent is the customary range for table service and 15 percent reads as dissatisfaction. Counter service, takeaway, and coffee shops carry much lower expectations.
Across most of Europe a service charge is often already included, and rounding up or leaving 5 to 10 percent is normal rather than expected. In Japan, South Korea, and much of East Asia, tipping is generally not practised at all, and attempting to tip can cause confusion.
The simplest approach is to calculate the tip on the full bill and then divide the total by the number of people. That works when everyone ordered roughly similar amounts.
When one person had a starter and water and another had three courses and wine, an even split quietly transfers money between friends. In that case, work out each person's share of the pre-tip subtotal, then apply the same tip percentage to each share. That keeps the tip proportional without anyone having to itemise the receipt line by line.
Rounding up to a convenient number is standard practice and rarely worth agonising over — the difference is usually less than the cost of the conversation.
Strictly speaking, the tip is for service, and tax is not service, so tipping on the pre-tax subtotal is defensible. In practice most people tip on the total shown because it is easier, and the difference amounts to a small fraction of the bill. Neither approach is likely to be noticed. What matters far more is the percentage you choose.
One thing worth checking: if the venue has already added a "service charge" or "gratuity" — common for large groups — you are not expected to tip again on top of it. Read the bottom of the bill before adding anything.