I am not sure but you can try this.
First, calculate new screen screen size with same aspect ratio as orginal. Lets say your curent resolution is 1280x720, and max width is 800, then your new resulution would be 1280:720=800:X => which is 800x450.
Second, calculate how much you must zoom out to achieve same aspect ratio of displayed objects. On orginal resolution 1280 layout scale is 1.0, your new layout scale would be 1280:1=800:X, which is 0,625.
edit: i messed up screen ratios first time