400 mg is lower than what some folks on this site use (I usually see 600, 650mg listed, I think people just get what they can find at the store) but so far it seems to do the trick for my oldest birds, who are sporadic layers but have thinning shells, so I was mainly looking to harden up their shells enough so they can safely get them out without breaking. But with a bantam that's a regular layer I would think a lower dose pill would be appropriate since they're obviously smaller.I'm in the UK so I'm not sure that our tablets would be the same dosage as in the US, so it's helpful to have that figure of 400mg.
Hope the tablets do the trick for your bird!