My first poolish

Hello everyone, this is my first post on the forum and I hope it's the first of a long series.

I'm trying to follow the txfarmer's recipe for poolish croissant. So, having done croissants a few times before but never had the chance to try preferments I decided to do some tests. I mixex 130g of water with 130g of strong flour (w350) and 0.5 grams of fresh baker's yeast. I wanted to see how long would the poolish take to be ready. Here you can see two pictures that show how the level of the polish rises in the first 8 hours, then it lowers a bit (you can see the mark left by the poolish) and then at 19.30 it starts rising again. So my question is when should I consider the poolish to be ready? My guess would be just when it's starting to go down for the first time. Is that correct? 

Thank you for your help!