I have a 7 gallon stainless steel brew kettle with weldless thermometer and ball valve. Measuring the water I put in the kettle is easy using a one gallon aluminum pitcher. However, once I get the boil going, It is hard to tell how much I have and if I need to boil it more to reduce the volume to 5 gallons (to hit OG) I tried marking my large stainless brewing spoon with a sharpie so I could put it in the kettle and see how much water was left during and after the boil, but those marks disappeared very quickly. I don't want to add a sight glass, because I know I would find a way to break it. Is there a good way to mark the inside of the brew kettle or my spoon? Is there a better way to measure the volume left in the brew kettle? Thanks