It'll vary because the Tiers vary. Not all of them starts with 100 and ends with 1000. My suggestion is about using 3/4 of the Tier, not 1.21212121212x from the Mountain level.
The Human level Tier fails because of the Tier itself, not the method. The method consist in using the + sign (wich is arbitrary). But you can use 3/4 of the Tier, without using the +. 3/4 of 1000 of the Mountain Tier is 750, it'd result in 1.33 times. The Human Tier would be 75 Joules. Again, it vary because the Tiers vary.
I suggested using based on the + sign, because it's something accepted, and it wouldn't be arbitrary. You can use the max of the Tier, but it'd be arbitrary.
But anyway, I'm in favor of using a mathematical solution with the use of Tiers.