I am using VB6 to capture a 60 Hz sine wave at a sample rate of 12288.
I am using a 120V outlet (with a DI-730) for testing. The sampled data
goes through FFT1024 where I get PeakIdx. With multiples of Peak IDX I
have the targeted hormonic frequencies. Using the PowerPt at these
indexes I get the FFT value of the frequency. To convert to dB I
divide by PowerScaler.

V = FFT10241.GetPowerPt(FFT10241.GetPeakIdx * X)/(FFT10241.PowerScaler)

Then I get the actual magnitude of the frequency by unscaling the value
(But I don't know what this value represents. Does anyone know? Can I
convert it to volts? )

A = 10^(V/10)

For THD I use the squareroot of the sum of the squares of each of the
values over the fundamental.

THD% = sqr(A1^2 + A2^2 + A3^3 ...)/A0 * 100

But this seems to be off (by just over 2X), I don't near enough the
same values as measured on a PQ Meter. I believe I need to compare the
harmonic magnitudes to the fundamental magnitude to get individual
harmonic content. I would like to get %, magnitude, and phase of the
individual harmonics. Am I on the right track? Can anyone help?