Primary fermentation with big mouth bubbler

Just purchased a 6.5 gallon big mouth bubbler. Upon bringing home and opening the bubbler does not have measuring marks. How is everyone using the bubbler topping off the wort to 5 gallons? Did you fill with water and mark the bubbler with a sharpie?

We went a gallon at a time, and I used some crappy nail polish that my wife hated to make marks.