Technical Question from a Technical Man

I'm a few steps down the path of my sourdough journey and would like a little clarity- I understand that I'm looking for between a 50%-70% rise during bulk fermentation (according to each individual recipe), but a rise from what initial point?

I have a straight sided Cambro that is marked for volume so it's easy to see; it's what I use for stretch and folds or coil folds- when should I take the initial measurement?