I got a sanke keg to get cut into a new keggle that ill be adding a ball valve and sight glass to. i I took out the diptube and it was just sitting around my house for a few days before i realized i could use it as a level gauge for my 8 Gal megapot. all i did was take my pipe cutter and ran it around the tube at the Gal marks just enough to bend it in a bit and make a nice level line. No sharpie or tape necessary I now have a nice stainless steel level gauge for my current kettle without wasting anything useful.
Im sure someone else has already done this and shared but i havent seen it so i thought i would. pic included.