Nevertheless, from my experience, the maths is extremely conservative for a balanced party played by veteran players —understandable, but frustrating.
Additionally, it doesn't tell you approximately what is the TPK probability of Hard encounter. According to the simulations it is 0–10%, which isn't that dangerous, especially as its an underestimate.
I might be scarred from Warhammer 40K: Dark Heresy, but I would say a Hard encounter is 20–50% TPK chance —a save by DM fiat is more fun than a last minute extra minion summoning to correct for the ease of the battle.
Having said that, I am unsure how it much could be improved apart from a few tweaks.