It's not an issue of my math improving. It's an issue of you not understanding how logarithmic functions work. The increase in T when CO2 doubles from 280ppm to 560ppm is calculated to be 1.2C. But we're already at 400ppm, and the increase in T so far is very close to 1C. The remaining 160ppm increase will increase T much less than the 120ppm to get us to 400ppm already increased it. Unfortunately I can't find the original paper I had read that calculated the temperature increase to be ~0.1C when going from 400ppm to 500ppm CO2, but the 1.2C per doubling will give you very similar results.