On the 90/180 Ukraine, I've heard from people staying in Ukraine for 5+ years now they just pay the fine (900uah, about $35USD) and your clear and reenter the next day if you wish. The 180 days is from the start of the first trip, so you would only have issue on re-entry if between 90 and 180 days and it is not a new calendar year yet (resets on Jan 1). So you need to overstay your free visa by at least 90 days each time if you need to leave come pack within the calendar year.
What a confusing way to state so much incorrectly. 
On 90 in 180 days ---- the simple way to explain it -- in the
LAST 180 days A new period resets daily ie a new 180 period starts every day.
To say it another way-- to calculate -you count days backwards up to the 180 th day -- and you are allowed 90 days in that period( including the arrival and departure days in the 90)
The only relevance of the first arrival date is that it starts the clock on a 180 day period.
Your comment on overstay is garbage -- in the past the fine has been relatively small -- and the ability to calculate was questionable.
Now -- as in more recent times-- the system is now computerised and the chances of an overstay going unnoticed is in a very low % chance.
Betting that an overstay will always be treated so leniently is a big risk-- the possibility for a 5 year exclusion is real-- as is a potential permanent exclusion. It should not be presumed that after being in breach that you are free to renter on the same basis as previously.
If you have read or been told otherwise -it is outdated info .