I put a 30lb dumbbell on it in the middle and got .005 inches of flex. I get about .001 inches when moving my z axis across it with no spindle on it so the question is what is too much flex and what should I shoot for?
That's something only you can answer. Different people have different expectations.

The bigger issue is that it's going to twist when that same 30lb load is pushing on it from the end of the Z axis.
And the sag or twist isn't the only issue. The fact that it can move, means it can vibrate as well, causing poor quality cuts.