1.) Using the CALC Widget
If you've added the widget to the Notification Center for a quick math without having to unlock your phone. You can just type a number + x% and it will show the tip to the left and a total bellow the expression inside the widget.
2.) Using the CALC App itself
Clear first (C/AC), then type in a number, you can then either swipe down on the percentage button or tap&hold it to show the tips. Since CALC v3.4.9 it will show you all tips and totals instantly - if you select one from the list, you will also see approximations of that percentage.
3.) Using the Apple Watch CALC App
Type in a number then swipe left to show instant tips, a split bill option and again approximations if you select any particular percentage from the list.