I've seen videos where they wait until the coals are white, some videos where the tops of the coals are kind of grey, etc. Based on your success, what does your starter look like the moment you put your steak on? Thanks!
When there is a layer of ash over the coals they are ready to go. You should see no black.
Another suggestion; get a metal bucket with a tight fitting lid. After you sear the steak dump the hot coals in the bucket and seal tightly. They will go out and you can use them over and over again.
I personally use this grill. Closing all the dampers extinguishes the coals and I can use them for multiple cooks. I always thought the chimney method was a waste when the coals are actually needed for such a short time. I also like that I can use it for a myriad of different cooking tasks.