Just above 70C. So if you want to be exact about it, I'd just practice boiling your kettle and testing it to get an idea of how quickly your kettle in your house cools off. Honestly, I boil a small amount of water (which will cool faster than a full kettle) and leave it for about 10 minutes. That seems to work well. When I first switched from BF, I was more precise and tended to wait 25-30 minutes, but after awhile, that's kinda a pain in the behind. Using a flask (thermos) to keep boiled water hot though is also great, so you might want to try that as well. Honestly though, it doesn't have to be perfect. As long as it's recently boiled but not actually boiling hot, you should be fine. Many of recommendations I've read seem to say to boil the kettle and wait 'just a few minutes' for it to stop boiling and cool slightly so you don't burn yourself. I think that's the main reason you're told to let it cool a bit (not the diminishing of nutrients). Formula manufacturers just don't want to be sued by exhausted mums burning themselves in the middle of the night trying to make a bottle.
