To get a rule of thumb cut-off at 3dB you need to find the noise floor of your instrument and compensate for this if it deviates from the hard-coded values too much.
We realize this is a bit cumbersome, but SNR is not giving the full picture either. Needed SNR to obtain good current measurements have decreased during the refining of these instruments through the years. Different conditions may require different SNR to give good readings.
A good check of the validity of the data is to check that the vertical velocity is close to zero. Current profiles that show large current variations vertically or vertical structures should be flagged suspicious. The ones that are most doubtful are those that are physically unrealistic (e.g. 1 m/s changes over a few meters). The measurements in a profile that are far from the mean are best removed.