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Atma is unfortunately very slightly incorrect on the Countess. Her drop is farily complicated, because of the fact that she has 8 potential drop chances but can only drop at most 6 items (like all monsters) and this makes her drop probabilities have a combinatorial aspect that is a bit beyond Atma's original scope. Atma will generally give you better odds than she actually has, because it counts the times when she might drop a rune 7th or 8th in her drop. However, with players=1, that is uncommon, so the numbers are pretty good at p1. They get worse as you raise the player number.
Good enough for a ballpark figure, however as BeB has since discovered, the chance of a Lem dropping is more around the 1/390 area than the 1/318 Atma claims.