Welcome to the forum, and thanks for the thanks – it’s members like you that make a forum work, so thanks for joining and being part of the discussion.
Regarding the Muse app: I haven’t used or tinkered with the Muse hardware or software yet, so I’m not sure I can be of much help. Based on the Muse FAQ, it looks to me like the feedback in the ‘Calm’ app is based on how long your brain’s been in a state that the Muse algorithm considers ‘Calm’. This means that, at very least, the app is using the duration beyond some threshold (such as level of alpha power), rather than just the current value. Probably the algorithm is more complex than this, and may involve the relationship between different bands over time.
The closest item in the Muse data might be the experimental ‘Mellow’ value (/muse/elements/experimental/mellow), based on the description in the documentation.
Possibly someone with Muse expertise (@yroy?) could point the way to a more informative answer!