The ratio between inlet and outlet area, in my limited experience, depends also on the "energy loss" into the hx (influenced by the porous properties and by the hx area). The optimised geometry you found at the end is about the same I used during 2016.
Well, ok for freezing the rules (but I am still convinced that 500mm would be more realistic than 450mm for our hx... anyway: it is only a small detail).