My kettle is a 40 litre stock pot. When I first got it, I wasn't sure whether the 40 litre was the actual total capacity of the vessel. I used a marked 2 litre container to fill it with water, and counted the number of containers needed to completely fill the kettle.
Guess what? It took exactly 40 litres to fill it.
Next step was to measure the distance from the bottom of the kettle to the lip. It was made easy, because it measured exactly 40 cm. So, using a simple ruler means each centimetre equals 1 litre. If it had been different from 40 cm, it wouldn't be difficult to work out what measurement equalled 1 litre.
I used a length of flat copper, and bent it so it hangs on the inside wall of my kettle. I engraved it with my Dremel tool at 1 cm intervals, and also engraved numbers at 10, 20, and 30 litres.
Easy to see how much liquid is in my kettle at any one time. Simple.