What should my reference value be when converting FFT bin amplitudes to dB?
You tagged this dbfs, so, that's basically your answer, because, yes, the output of a DFT is very much predictable! 

What you choose, however, as reference amplitude is up to you to define as useful to you.  I can but make a recommendation:

Assume you're transforming a constant vector of $N$ maximum (ADC) values; that'll give you, depending on the normalization that your DFT applies, an all-0 vector with the equivalent energy in the DC-bin. That'd be a useful value for the full-scale maximum; frequency shifting and superposition with other signals can never yield a more powerful bin.