Firebase rules .validate returns: Permission Denied

I am trying to set a value in the database which validates in the rules. I want to make sure that the score been set is actually the score.

The new score added to the database should be:

initial score * the players level + their current score.

Here is my database rule:

    "scores": {
        "$user": {
          "score": {
            ".write": "auth.uid === $user",
            ".read": "auth.uid === $user",
            ".validate": "newData.isNumber() && newData.val() === ((root.child('initial_data').child('score').val() * root.child('users/' + auth.uid).child('level').val()) + root.child('scores/' + auth.uid).child('score').val())"
          }
        }
    }

and I am calling the write function like so:

    var data = {
        balance: score + (initial_score * level),
        last_updated: last_updated
    }

    var ref = fb.ref().child('scores/' + userId).set(data, function (err) {
        if (err) { console.error(err); }
    });

When I console log data I get:

{score: 2376, last_updated: 1566372843943}

Here is the data in my database:

{
  "initial_data" : {
    "score" : 200
  },
  "scores" : {
    "h3***********FAn2" : {
      "score" : 376,
      "last_updated" : 1566231627824
    }
  },
  "users" : {
    "h3***********FAn2" : {
      "level" : 10,
      "timestamp" : 1566226654955
    }
  }
}

and in the console I get the error:

Error: PERMISSION_DENIED: Permission denied

at Repo.ts:632

at Et (util.ts:585)

at ci.callOnCompleteCallback (Repo.ts:624)

at Repo.ts:356

at PersistentConnection.ts:523

......

1) Where am I going wrong and how do I fix it?

2) I think the issue is with the root.child() not returning correctly. Is there a way to see what this evaluates as in firebase console?