Percentage scaling

[Ofnuts]

I assume you are talking about zooming and not scaling.

Starting with this:

   

Have you tried: 

• View > Zoom > Fit image in window: the image is zoomed to fill as much of the window as possible, while remaining entirely visible
  

   

• View > Zoom > Fill window: the image is zoomed to completely fill the window so there are no margins around the canvas, but one dimension is not complete 
  

   

If needed, you can assign keyboard shortcuts to these functions.