Personally, I have a sticky note-esque app on my phone that I've been using. I write down the date and time of each emptying/reinsertion, as well as an eyeballed amount ("half-way between lines," for example). I also like to add why I emptied it--whether there was a big leak or it was just a routine change. Finally, I jot down how I reinserted it: "punch down fold facing back/right" seems to be my preference. However, I'm looking around to see if there's a way to streamline it. My phone system is okay for now, because I've been working 10-hour days without my laptop and I can always add what I need to, since I always have my phone with me. Ideally I'd like some sort of online calender where I can track spotting (without a cup), my whole period, and cramps that I sometimes get during ovulation.
(I apologize if this has been asked somewhere--I looked around and couldn't find it. I'm also not sure which tags would be most applicable for this question.)