I may be against the mold, but I think crowdsourcing actually works best here to fine tune numbers. Maybe not having the data on the box, but instead on a website that is a collab between the industry and something like HLTB. This allows for the times to be fine tuned based on how people are playing the games. The beauty of crowdsourced times is they allow people to share their experiences about how their Main Story+ playthrough went and what they valued.