If using a weldless fitting set.
Use a sharpie for marking (alcohol will remove the marks)
To get the fitting all the way to the bottom, put your washer inside the pot and hold it so the washer is close to the bottom and still flat against the wall.
Make a mark inside the bottom of the hole of the washer and then measure from the lip of the pot down to the mark.
Make a mark on the outside of the kettle from the lip down to the same measurement.
This will the be the bottom of the hole for the nipple. Hold the nipple with the mark on the bottom and trace around the nipple to get the hole you will drill.
Mark the center of the hole, use a center punch on the mark, and drill your hole.