I was playing around a bit in multiplayer mode alone in a private game to try out some stuff and noticed that I don't get any improvement in galactic readiness if I fail in one of the early waves.
So I wondered how many waves I do have to survive to get some increase to galactic readiness? I assume that I only get the full bonus if I finish all waves, but from which wave on do I at least get a partial bonus? And how much of the maximum bonus do I get at each possible step?
I did a quick experiment myself and didn't get anything for dying during the third wave, so there seems to be some minimum requirement before you get any increase in galactic readiness.