I just passed scale to xrandr after computing the proper scale and then used the nvidia-settings gui to write current configuration to xorg.conf its not incredibly hard basically all you are doing is scaling lower DPI items up to the same resolution as your highest dpi item and letting it scale down the correct physical size. For instance if you have 27' monitors that are 4K and 1080p you just scale the 1080 ones by 2 if you have a 4k 27 and a 1080 24" its closer to 1.75. The correct ratio can be found with your favorite calculator app.
You can set this scaling directly in nvidia-settings come to think of it where you set viewport in and viewport out.