Custom output width dynamic relative to portrait or landscape

[Craigjw]

Using the option to use a custom width, Portrait and landscape images are upscaled to different total resolutions
Can a dynamic option to have the image upscaled to the desired pixels dependent on portrait or landscape.
currently using width = 1600
800 x 600 = 1600 x 1200
600 x 800 = 1600 x 2100

suggested dynamic:
800 x 600 = 1600 x 1200
600 x 800 = 1200 x 1600

I hope that this makes sense as it'd be a really useful addition.