As low as is currently feasible to get it at the point of this theoretical war, and lower once we have the capabilities to make it lower.
Because this is an evolving standard based on our current capabilities and the stakes of the theoretical moral war you propose, there is no single universal answer, we'll always be paying attention to the context and we'll always be adjusting. It is thus very important that the public have information about what the current casualty rate is for civilians. There is no easy, universal numeric answer to what is an acceptable casualty rate, but in no world is a secret percentage of collateral casualties acceptable. The public needs some level of access to what is going on so we can have informed debates about whether what we're doing is justified.
The Intercept's original coverage of this leak wrote:
> The implicit message on drone strikes from the Obama administration has been one of trust, but don’t verify.
> [...] Additional documents on high-value kill/capture operations in Afghanistan buttress previous accounts of how the Obama administration masks the true number of civilians killed in drone strikes by categorizing unidentified people killed in a strike as enemies, even if they were not the intended targets.
How could the public have a meaningful debate about the morality and acceptability of drone strikes when we didn't even know what their effects were, when they were being misrepresented to us by our own government? You're asking for a single, universal number of what is and isn't acceptable. I'm asking just to know what our current rate is, and to not have the government lie to me about that rate. We can't debate whether or not our results are acceptable if we don't even know what our results currently are.
There are possible wars that would only be moral if, and only if, we could ensure some level of collateral damage or less. Perhaps it would be moral to wage war over X, but only if we could guarantee there would be no collateral damage. Conversely, if some country of psychopaths were doing some unspeakably horrible thing Y then it may be moral to stop them even if stopping them would require us to kill them to the last man, woman, and child. We would need very different casus belli to justify a moral war depending, in part, on the amount of collateral damage prosecuting that war would cause.
You will now perhaps attempt to argue that this does not constitute “intentional” killing of innocents, which has a similar problem with relevance. Indiscriminate killing is highly immoral.