A while back, I took 102 Tales of Terror and tracked my progress in converting them. Short version: If you want to make Echos to grind your Overgoat, Hunter’s Keep is still the place to be. That said, the results:
102 Tales of Terror converted to 84 Senses of Deja Vu. This was tracking higher up until the last 30; I suspect it has a 90% success rate, but it may be 75%, 80%, or 85%.
84 Senses of Deja Vu converted to 64 Glimpses of Something Larger. About 75%
64 Glimpses of Something Larger converted to 48 Deals with a Devil; exactly 75%.
48 Deals with a Devil converted to 26 Room Numbers at the Royal Beth; about 50%
26 Room Numbers at the Royal Beth converted to 12 Last Hopes of a Fidgeting Writer; about 50%
12 Last Hopes of a Fidgeting Writer converted to 8 Lenses of Black Glass; this could be 75%, 50%, anywhere in between. I’m inclined to guess lower, since that’s been the trend and it’s labeled “it could go either way” rather than “pretty good odds.”
8 Lenses of Black Glass converted to 4 Coruscating Souls. 50%, but not a meaningful figure. Notably, this also states that it “could go either way”
In the end, with optimistic assumptions of {90%,75%,75%,50%,50%,50%,50%}, the ending value of 312.5, and a starting value of 0.5, with the returns spread out over 7 turns, this gives you 0.91 Echoes per action, before subtracting the additional items you need to add in to convert up the chain. Even if the Last Hope -> Lens ratio is 75%, it’s only 1.62 Echos per action before taking away the additional items.
Hunter’s Keep, by contrast, is 1.27 without the Searing Enigmas, and 1.45 assuming the chance of a rare success is about 10%, though it requires a 30-action lead-in that operates at about 1.2. Unless the chance of a Searing Enigma is very small, it is still worthwhile to Zail to the three ‘sisters’ to farm your Overgoat.