tempdeltacalcs.js

{
  $let: {
    vars: {
      targetdoc: {
        $max: {
          $filter: {
            input: "$location_lookup_targettemps",
            cond: {
              $gte: ["$$this.to", "$miltime"]
            }
          }
        }
      }
    },
    in: {
      $subtract: ["$average", "$$targetdoc.target"]
    }
  }
}