[Mr Ramsbotham]

400 ppm seems a bit low.

[Carriage Hill]

Ranging from 310ppm (1958) to 405ppm (2014), it's been based on the Keeling Curve, taking measurements in Hawaii, California and Arizona.

[Morpheus2009]

Most plants will actually do just fine at those levels, in fact CO2 is the limiting factor for photosynthesis.