CIV "standard setting" is: normal speed, normal size continents (or possibly fractal) at normal sealevel/etc, standard boxes, random civ, random opponents (default # for map size), no regen/no reload.
I personally consider a level mastered when I win 10 games under those settings in a row, I did that a long time ago for vanilla immortal, and I fully expect to win at least 90% on BtS immortal.
Vanilla/Warlords deity was a different matter tho. The best Warlords deity Apollo dates I've seen beat BtS deity worst Liberalism dates I've heard of, go figure. I've also seen vanilla/Warlords deity lead me with dozens of techs, get Liberalism in the BCs, insta-upgrade to longbows and then to riflemen at impossible dates, settle cities all over in no time, have its cities grow to 20 and then 30 at even more impossible dates, have hundreds and hundreds of units on normal size maps, you name it.
I learned a lot about rushing, blocking, squeezing the most out of little land, the technology catchup game, leveraging a minute military advantage, managing diplo, and surviving when diplo fails while playing Warlords. For all that, I have no "standard settings" games won on vanilla/Warlords deity, at the very least I selected a powerhouse civ like Inca/Egypt/Persia/Rome or Liz England.
BtS deity is harder than any Immortal, yet a lot more forgiving than its pre-Blake AI counterparts.
Take for instance the Dual Deity Challenge - a lot of players won at least one of the games, some won both. Granted, Epic made it easier, and Tectonics might have helped too, but it wasn't far removed from "standard" and we had many wins.
Then we had the Ultimate Warmonger challenge: stuck on pangea with all the aggressive warmongers, and again we had winning submissions. In my game in particular, I rushed an AI, fought off two others, completed two wonders and two national wonders, got Music first, got tech parity/in position for tech superiority by the early ADs...back on Warlords it would be unthinkable to simultaneously win wars v multiple AIs, complete wonders, and get ahead in tech.
That's why I believe 90% victory rate on "standard settings" BtS deity is within the reach of "perfect human play".