It's a logarithmic scale. The 0.73 value is not a linear scaling but a value given by Sagan's proposed interpolating/extrapolating function: K = (log10(P) - 6

It's a logarithmic scale. The 0.73 value is not a linear scaling but a value given by Sagan's proposed interpolating/extrapolating function: K = (log10(P) - 6) / 10, where P is power in Watts. 20 TW = 2e13 W -> K = (log10(2e13) - 6) / 10 = ~(13.3 - 6) / 10 = ~0.73