The key would be to "notch" the plastic or metal rod/tube with, say, a turn of the pipe cutter or kitchen knife. Numbering the notches is more difficult--a dremmel might be the best tool. From personal experience, I can say that a black sharpie does NOT work. The wort pretty much dissolves it right off of a plastic mash spoon. All the time spent adding a gallon at a time to the BK was wasted. Plus, black sharpie stuff got dissolved into the wort--not necessarily what one wants in the beer.