I can't find any digital samples of this doujinshi so I changed the colors by feelings.

Feelings? I use the histogram.

lol. Yeah I would adjust the black level based on histogram to prevent clipping the color. But I also won't clip them all which makes them too dark..

for this book, I use input 52, 255 and gamma = 1.05

i'm used to using samples to adjust r/g/b, I mean.

Color seems over saturated to my eyes. I know a lot of books from this artist tend to have that raised black floor that makes everything washed out, but if you pull the black down it makes everything look gaudy and compresses the mid-tones too much. Just my .02 cents for whatever its worth (probably less than that).

When I fix images like this for my personal collection there is a layer mask trick I use for contrast enhancement that works really well without blowing the colors out the way level adjustments will do.

most of scans are saturated than digital one.

and what's the meaning of "compresses the mid-tones"? if I "raised black floor", the midtone should be extended instead of compressed.

Just a personal perception, maybe I'm not expressing it right. To me when you adjust the levels the way you're describing it compresses the range of remaining visible colors so it looks like it's all dark tones and light tones with a lack of tones in the middle. I think auto tone in photoshop makes everything look like that, if I need to adjust levels I usually am more subtle about it than whatever it suggests but it depends on the individual image of course.

I used to over contrast everything to death, but over the years I've found I don't like that look anymore.

I can guarantee you all know more about it than I do, I comment mostly so I can strike up dialogue and learn.

In general, contrast is decreased while printing, mostly in shadow tones, while saturation is raised in neutral colors (depending on the colors papers have). Google doesn't know this (at least I can search) and no one solved this matter.