A 455 grain arrow is what I have shot for many years. It is on the heavier side, but not considered heavy- per se'. Especially when most folks obsessed with speed are shooting 50-75 grains less.
In archery hunting lethality is about deliverable energy, shot placement and penetration. Penetration is all about delivering KE. So, as stated previously, there are several factors assuming your form and accuracy is good. Lethal KE depends on the type of game you're hunting in a big way, I might add
1.Small to medium game- antelope- whitetails requires @ 55#-65#
2. Large Game - Elk Moose 65+#
3. Dangerous game 70+#
There are charts available for that information but the above are deliverable KE I'm comfortable with, though they're all a bit overkill. In my opinion, an animal can't be killed too effectively. It's all about being quick and ethical
KE Mathematics Formula:
Kinetic energy is equal to arrow weight, multiplied by arrow velocity (squared), divided by 450,800
Momentum Formula
Momentum (arrow) = mass of arrow (grains) x arrow speed (fps) / 225,400
There are charts available for that information but the above are deliverable KE I'm comfortable with.
Edit: I don't focus on FOC as much as KE and Momentum. Get your bow and form tuned to delivering the right KE and you'll drop what you aim at.