@Hold Fast No......you need to be flexible, and not necessarily...

@Hold Fast

No......you need to be flexible, and not necessarily exact (because 'where do you draw the line ?? ' ).......just do it by eye.
Just look at the bar in question, and compare it to the other bars, and in particular the most recent bars.
It should be quite obvious if the bar is "really narrow"....if it is just a little narrow, then it may or may not be interesting, and only future price action will confirm.
The main thing you are looking for is an anomaly between spread and volume.
Normally volume and spread are a proxy for each other, so when volume increases so does spread (and therefore volatility, because in the end spread is volatility), and when volume reduces, so does spread. But in particular - when volume increases a lot and spread doesn't, then sit up and pay attention, & watch for the next bar to respond & confirm.

cheers