Estimating harmonic loudness
It would help if you could state what exactly you are trying to do. 
The "relative loudness of the harmonics" is normally a function of time because of the different decay rates and/or temporal envelope of the harmonics. There are different ways to condense this into a single number: initial amplitude, peak amplitude, total energy, 250ms into the decay, etc. The most suitable method would depend on your specific goal.

The look up table looks questionable to me. None of the loudness curves that I'm aware of (A-weighting, C-weighting, Fletcher Munson, Robinson Dadson, ISO 226) look like that. -9dB at 1500 Hz seems just plain wrong.

The shape of the loudness curves is a function of absolute level. So whatever curve you pick it can only be correct at one level (or a small range of levels). In order to get this really right, you would need measurements that are calibrated in Pascal or dBSPL.