I have to say that I'm really loving my new Weck jars for feeding my starters with. Once fed, I can mark the jar with a Listo Marking Pencil (grease pencil) or a White Board dry marker where it sits in the jar along with the time. Either pencil marks are easily erasable with a damp cloth.
I find that marking after each feeding and watching the duration of the rise tells me how strong my starter is becoming.
Always interested in making the sourdough challenge easier!