Oh, I didn't notice the negative. So basically x/(x+1)? That works for a base damage of 1 per round and burst count of 2, yeah. But in general, it should be x/(x + damage_per_round), at least for the lower bound of the ammo efficiency. Which in the case of Battacor is x/(x+66).
Not sure what you mean by "perfectly" though. The ammo required part is an integer, not a fractional amount, if that's what you mean. E.g. for an enemy with 50 health, you'd need to first 1 round, though you actually fire 2, so you get an efficiency of 50%. Theoretically, you could say you only need to fire 50/66 = 0.758 rounds, though you still fire 2. Which will give you a lower efficiency. In this case, the lower bound would actually be the same as the damage efficiency's lower bound, x/(x+131).