Technical term for percentage of CO2 remaining in the atmosphere
We call this the airborne fraction, although as the name suggests, it's normally expressed as a fraction rather than a percentage.

Raupach et al (2014) is an (open access) example of it being used in the literature, and in that paper they quote a long-term 1959 to 2012 value of 0.44, so in the same ball-park as your estimate.  Similarly, here's an example of it's use in Chapter 7 of the IPCC AR4 with a range of modelled values from 0.4 to 0.5.  It's also often calculated over single years, as in your example, to look at the trends and variability.

As an aside, the 2 ppm in your calculation is called the atmospheric CO2 growth rate and the total mass of CO2 in the atmosphere would be the atmospheric CO2 burden.