reference value for calculating dB from FFT-values

I need some help,

what is the reference-value, to calculate dB from values
returne by FFT?

Normaly, the SPL is L=10 log(Ix/I0) with I0=10E-12 W/m^2.

In the MPEG-Source code, all FFT-values should be normalized to the reference-level of 96dB SPL, but how do I know, what the value calculated from FFT is, when I don't have the reference-value.

Thank You for help me
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