The Framing Tool is a little clunky - you have to select "Custom" Crop in the first drop down menu, select "1920x1080" in the second drop down menu, and Set Fit to "Fit Width."
To output 4.5k in a 1080P with letterbox, you only need to select "Fit Width".
In order to have accurate cropping to the pixel, a crop must be specified based on the source footage resolution (second menu). Without that, the crops aren't accurate which is why the framing is setup that way.