Watched my old boss do this for years at her salon. She had this whole wall of toners and would literally mix a drop of this and a drop of that based on how the client's hair was pulling that day. I always thought she was just being extra, honestly, because the rest of us were grabbing the same bottle every single time. Then one day a regular came in with hair that had gone way more yellow than usual from a new well water situation, and my usual toner did basically nothing. My boss walked over, looked at it for two seconds, mixed something custom, and it came out perfect. That was the day I stopped thinking of toner as a one size fits all thing. The girl you talked to might be young, but she's not wrong about reading the hair fresh each time.