I would say shoot for 450 but 400 is not going to harm them.
At least at 450 you have room for it to drop a little. Anything under 400 is starting to get a bit low.
Don't forget your Alk and Mag. Keeping your mag in check will allow your alk and calc to get high enough. Low mag and you won't be able to get the other two up to where they should be.