What is the turn requirement to get a S rating in conquest?

Can anyone provide a guideline of the turn (and city) threshold for S and A rating per country and year? For example, a 3-star nation in 1798 needs to complete within aa turns and capture bb cities for a S rating and cc turns and dd cities for an A rating. A 2-star nation in 1806 needs to complete within ww turns and capture xx cities for a S rating and yy turns and zz cities for an A rating.

I find it quite frustrating that I have zero clue on what to aim for and simply have to resort to "try your best and hope for a good rating." Having received an A rating for 1798 Prussia, a 2-star nation by completing in 99 turns and capturing 52 cities, I thought I was in for a poorer rating when I completed 1806 Sweden, a 2-star nation in 156 turns and captured 75 cities. After all, it took 58% more turns, right? However, to my surprise, I got the same A rating.