Scaling external Display on Macbook Pro Mid 2012
You're best off using Mini DisplayPort to DisplayPort cable for this.
  HDMI would work but you'd need to make sure your mDP to HDMI adapter
  and HDMI cable were compliant. It's just easier to buy one mDP to DP
  cable.


The easiest way is to hold Alt when you click on the Scaled option, which will force macOS to show the additional scaled resolutions in the list.

Here's Displays for my Apple Cinema HD Display as standard:


And then after Alt-clicking the Scaled option:


Alternatively, you can use a menu bar app called RDM (download link at bottom of readme) which lists all available resolutions, with retina resolutions marked by the ⚡️ lightning bolt emoji: