Regress nonlinear equation

[scantor]

Below is a nonlinear model, together with calibration data for which I would like to find the parameters Ro, Kc, a and b.

R = Ro + Kc * (1/(1 + exp(-(a + b*lnC))

Calibration Data

C R
0.0 0.3347
4.8 0.4201
7.7 0.5141
11.2 0.6177
14.4 0.6887

Given the calibration data above, how can I calculate or at least estimate these parameters?
I do not want to use an add-in program, like Solver, in Excel. I do, however, want to be able to use any existing functions, if necessary, in Excel. i'd appreciate it if someone can walk me through the steps.

Thanks,

Sheldon