Matching an LPC Magnitude Spectrum to FFT Magnitude Spectrum
I have a similar function to compute the Spectrum from the predicator coefficients, but instead of your code that I do not understand, I have the following :

pOutSpec[spec] = mMeanSquareError / (sumR * sumR + sumI * sumI);


I have something that computes the real and imaginary parts into my buffer :

data[2 * spec] = sumR / (sumR * sumR + sumI * sumI);
data[(2 * spec) + 1] = -sumI / (sumR * sumR + sumI * sumI);


Then I use different functions to display the spectrum according to the chosen format (Real, Magnitude, Square Magnitude or Log).

I also have some problems with scaling, both for the FFT Spectrum and the LPC Spectrum. I have chosen to scale them based on my display area, but as a result, their scale is not exactly the same on the display.