Beside using numerical pain rating scale, for low back pain, the best way to evaluate is with functionnal questionnaires like Oswestry or Roland-Morris.
I think using qualitative study may reveal other aspects of LBP that can lead to develop new tools in this field. You can also design an online interview in situations that direct interview is not possible.
I dont have an actual scale, but I try and direct my clients to a comfort scale. Based on Function, such as, how long did you notice comfort while sitting? How easy was it to get out of a chair today? How was it walking with ease in your step? Not easy to quantify, but moves the measure towards a cognitive behavioral model and less about pain measurement.
I agree with what was said above and you might also want to consider looking at functional ability based on measures used by physical and occupational therapists.