I'm not sure what you mean, or why you say this.

I mean that the military administration of the Manhattan Project wasn't actually equipped to deal with existential risk calculations, the scientific side of the project would have known this, and the administrative side would have known they'd known. It's effectively a technical obstacle and would have been dealt with as such.

In actuality, that question resolved itself when further investigation showed that it wasn't going to be a problem. But if the answer had been "no, we've done the math and we think it's too risky", I think that would have been accepted too (though probably not immediately or without resistance). I don't think that flat percentages would at any stage have been offered for the interpretation of people not competent to interpret them.