How do you know that you have a 50% stock reduction when you cannot see the bottom of the kettle? This can be done with a story stick.
You cannot very well mark the side of the kettle for level.
I use the dowel part of a wooden spoon to measure the level of the liquid before boiling, dip the dowel into the liquid and them mark with a polymer pencil the 100% level.
You can also create a calibrated story stick that would belong to kettle. In this case, in a dry kettle you would mark a line 100% to the rim of the kettle. Then add known volumes to the kettle either in quarts, gallons or milliliters and make a mark on the story stick.
A story stick can be very accurate.