Something like this should be good enough. For custom gamma I'm sure there is a set number that people will use to make the contrast look better for manga but I'm unsure of what the number should be. If you don't like how some manga look you can always fiddle around with the custom gamma options. I saw some user mentioned that he adjusted the number to 2.50 for his Kobo Clara Colour which seems odd.
Hover over the spread splitter option and see which one you'd prefer more.
I'd highly suggest installing something like KOReader for your Kobo device since it's easier to adjust the contrast within the interface. This is also a good way to separate your ebooks vs your manga library. The benefit with KOReader is that you only need to store your manga into their respective folders without the need to fix up any of the metadata which is quite good.