Is this the correct way of making the stacksize to 1.5x?

{
  "Settings": {
    "CategoryDefaultStack": {
      "Ammunition": 0,
      "Attire": 0,
      "Common": 0,
      "Component": 0,
      "Construction": 0,
      "Food": 0,
      "Items": 0,
      "Medical": 0,
      "Misc": 0,
      "Resources": 0,
      "Tools": 0,
      "Traps": 0,
      "Weapon": 0
    },
    "DefaultStack": 1.5,
    "StackHealthItems": true
  }
}

If not, mind copy-ing that a showing me what's there in change? Thank you

No, the default stack is not a multiplier. It has to be a real number, and applied using the stackall command (/stackall default).

You will most likely get an error, as the plugin does not anticipate a float (1.5) in place for an int (1)