How exactly are the ratings calculated?

i'm not sure what you mean by public rating and personal rating,i dont think such a thing exists on this site.
Nonetheless you have a better chance of getting answered if you ask this at the support forum.

 Mimi:

By "public ratings" I mean those you publish for others to see. By "personal" ones,  I mean those only you can see. 

Ty btw for the answer 🙏🏻

there are no rating that only you can see. All ratings of all viewers can be seen on their dramalist page. I am pretty sure there is no way to hide the rating. 

Yeah as of now review ratings do not count for the average rating on the drama page. Only ratings given next to the title on dramalist count. So the "personal" ones.

The reason for review ratings not to count, is the fact it would mean some users would have 2 ratings influencing the average on - the one from dramalist and the one from review. 

This being MDL, if a user liked something, it tends to be a 9 or 10.  No-one gives anything less than a 5 unless it was absolutely hated.

But I do believe ratings are based on user votes.

My personal rating system - which I've posted several times (and I really should add to my profile, but I can't see where to alter it) goes something like this:

1: The worst.  I probably dropped it but it's an epic failure in either storytelling, technical abilities and/or both.  It's worth noting that blatant propaganda might also fall under this figure.

2-3: Various levels of suckiness.

4: Under-par.

5: Middle-of-the-road.  Honestly: the majority of stuff released worldwide is this or...

6: Slightly above mediocre; only worth your time if you're a die-hard fan of the genre and/or someone involved in the project.

7: Good.  Hell, recommendable even,

8: Very good!  Definitely recommended.

9: About the highest rating I can give a title, it hit all the right notes and left me looking for something similar... even though I know there's precious little that can follow it.

10: Perfect.  Or, well, about as good as a film/series can be.  Almost nothing ever gets this rating.

Oddly enough, people have called me out for rating titles a 6, as if I'm cussing their favourite show and, by extension, them!
People need to grow thicker skins (and be more open than having a dichotomy of either loving or hating something).