I guess one must define "adequate" first. The default value of 80% could work fine if you are developing a "find your celebrity doppelgänger" game while law enforcement should probably use 99%.
I think you'd have a point at which the threshold increases but the probability of the true positive being in your results starts to drop severely. 99% might be useless if you have one or two hits and they are unlikely to be correct. You can't assume that the one you're looking for will be a 100% match; if it was, then you'd just set the threshold to that, presto.
