I tried 63 attack and 9 lethality vs. Spartans. First volley = 2 kills.

I then tried cutting lethality to 0.5. First volley = 4 kills.

Conclusion: The difference in kills was random variation against a 2 hit point unit (where two hits and kills must register on a single man in the first volley.) If the 9 lethality worked at all it would have killed at least a dozen spartans.

They were mown down like wheat in subsequent volleys since almost all front rankers had lost their 1st hit point.