How can I get the correct ttf font size based on simulator.json?

I'm trying to match the datafield label font in my DF (based on DataField class) For older devices (i.e fenix6) it used to work, because in simulator.json there are things like:

{
  "layouts": [
    {
      "datafields": {
        "datafields": [
          {
            "label": {
              "font": "xtiny"
            }
          }
        ]
      }
    }
  ]
}

So I use FONT_XTINY.

(For the sake of this post I'm looking at the "ww" variants)

But on newer devices (i.e fr965) no it uses ttf:

{
  "fonts": [
    {
      "fonts": [
        {
          "filename": "Roboto-Regular",
          "name": "simExtNumber2",
          "size": 5.4424,
          "type": "ttf"
        }
      ]
    }
  ]
  "layouts": [
    {
      "datafields": {
        "datafields": [
          {
            "label": {
              "font": "simExtNumber2"
            }
          }
        ]
      }
    }
  ]
}

However when I use RobotoRegular (BTW: IMHO there's a bug in simulator.json, and the filename probably should not contain the '-', but that's another issue) with size: 5.4424 then it is way to small:

var labelFont = Graphics.getVectorFont({:face => "RobotoRegular", :size => 5.4424});

(Note the datafield label is above "searching .")

Interestingly when I look here: https://developer.garmin.com/connect-iq/device-reference/fr965/ then there are different numbers from what is in simulator.json. i.e:

xtiny: simulator.json: 6.9542, device-reference: 37

Does anyone know how to "translate" the size in simulator.json to a number that when used in getVectorFont, will look like the real label?