New to sourdough!

This is my second attempt. I’m happy with the oven spring but haven’t cut into it yet as I think I should let it cool (I read I should wait for 8 hours!). I have a question about bulk fermenting. I’ve been reading about the ‘science’ of sourdough and it’s all very interesting. There is one you tube chef who makes wonderful bread who puts his dough in a bulk fermenting container - ie after he has done all his folding and resting, so it’s already been a couple of hours since he added his levain to the mixture. Which is when I understand the fermenting begins. He then gauges when to stop his bull ferment when the dough has expanded by 25-50%. But this seems arbitrary to me as at least a couple of hours fermenting has already taken place. I saw someone else on you tube who puts a small amount of dough in a tiny pot right after incorporating the levain into the dough, and bulk fermenting is complete when it’s doubled in size. Which for me is a better method (if it’s an accurate method) as my kitchen is quite cold in winter and very hot in summer so fermenting ‘times’ might be a bit hit and miss. Any advice on assessing fermenting times would be most welcome. Also, I prefer a darker loaf...have I overdone it here?