[mci]

If your metric is the length of the land that the map boundary cuts, then the best centre lies somewhere in the Indian Ocean. Proven here: [1]

[1] https://marcinciura.wordpress.com/2015/11/17/slicing-earth-c...

[hoseja]

I seem to have found this one, basically: https://marcinciura.wordpress.com/min-land-4/ But my metric wasn't as much intersection as maximum land area in one half.

edit: I didn't even know about this: https://en.wikipedia.org/wiki/Land_and_water_hemispheres