I used my 9 gal stainless brew pot for my first all grain this afternoon. Because I wanted to know when I was at my target 6.5 gal wort level and then my 5 gal target level, I did a little search and found this website for calculating the depth of liquid for a particular volume and radius pot.
Cylinder Calculator
You will have to convert gallons required to cubic inches (multiply by 231) to get the required depth in inches.
I then took a blunt screwdriver and hammered a very slight indentation on the outside of the pot at both the 6.5 and 5 gallon marks so that is showed though on the inside of the pot. It saves measuing with liquid and wasting water!
Hopefully someone will find this useful.