In DPL4 (and DPL3) on macOS Mojave, I can enter any ratio, no matter if the pulldown is open or closed. The ratio I enter persists and the crop shows as entered: width:hight.
For A3 (landscape), I enter 420:297, the exact measures in mm. To take into account a border of 10mm (12mm at the bottom), I can then simply enter 400:275…