Shoe size chart question

I would love some clarification on what people mean when they “size down 1 size from street shoe size.”

I wear an 8.5-9 US women’s. In my street shoe experience, that’s usually a 39.5-40 in Europe and marked on the shoe. But on this LS climbing shoe size chart, a 8.5-9 is a 40-40.5. So where am I starting before I think about sizing down? Do I start at the US size and then size down based on the Eur equivalent size column? It’s tough because there aren’t many climbing shoe options in my area so I have to play the order 2-3 pair and return game. Ty!