Obtaining a high sample rate digital approximation to an analog filter from lower frequency measurements
You mention $2F_{a_{max}}$, which makes me think that you're trying to invoke the Nyquist-Shannon sampling theorem.  But that theorem doesn't deal with any "highest measured values".  It deals with that process of getting a signal from the continuous-time domain (or some higher-rate sampled domain) to a sampled-time domain.

I think if you have an existing filter that you want to measure and duplicate, that your best bet would be to reverse-engineer the analog filter transfer function, and then use any of the numerous filter design methods to design a digital filter that replicates whatever part of the filter behavior you want to replicate.

Note that unless you sample really fast compared to your highest frequency of interest, you're going to have trouble (or not manage to) get both the phase and the amplitude response to match.