Okay, making this post mainly to acknowledge the Persia topic. Because without looking at it myself I can't really say a lot that is well founded.
From a gameplay perspective, 8% of territory is fine as far as challenge goes. Historical accuracy is not that important because it can always be justified as a "surpass history" goal, such as trying to establish control over Greece, or as an alt-hist scenario, Upper Egypt or the Indus Valley. Furthermore, the 8% take into account that you are generally expected to control all cities within the historically conquered area, but in game their culture usually allows you to cover more.
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