Thank you very very much man
I've found a different way of making it into black and white, but I also thank you for that answer and the other ones.
The mathematical equation got me a bit confused at first until I realized I was using km/px instead of px/km, after that though, it was pretty easy.
And then regarding the scale part:
Is it used only if you wish to make a printed version of the map or can it be used digitally?
I also facepalmed myself so hard after you said that it was a scale, quite likely the last thing I would have ever thought of.

And also thank you for the river tip.
I never really researched the subject of river placements before since it was not entirely in my train of thoughts when I was looking at the unfinished map.
Again, I thank you for the answer to my questions and regarding the river placements

Cheers.