Providing a personal answer to this question is basically impossible for me because
1) I do not use XPs
2) I do not use level as an indication of mundane skill, but as an indication of how much the person knows about the hidden workings of the world and how much they have actually experienced those.
3) the level spread is not regular, i.e. there are glass ceilings and cutoff points - in other words, doing the mundane, even if you strive to better yourself, does not get you past a certain point.
But hypothetically, I would use the common soldiers of the military charts provided in the setting to determine average "level" and corresponding proficiency. Not the guards, elite soldiers or other exceptional individuals, mind you (since those are, by definition, exceptions to the common rule). My reasoning is that, if your average soldier (whose job puts their life on the line at least occasionally) is level 3 or 4, it stands reasonable to believe that those who do not have a more serious incentive to become more proficient at what they are will likely be at the same level or lower on the average.
Mind you, this is for an *average* soldier - probably garrison type who sees combat infrequently, but trains pretty much everyday. Your weathered veteran mercenary will be of higher "level" because they are on the edge more often, simple as that.
But your average city merchant or craftsman ? Level 2-3 seems a fair average, regardless of age. The basic criterium is not what you do routinely, but how often you are on the edge. The more often you are, the higher "level" you can potentially be - if you survive the experience.
Basically, this was my rationale to change the meaning of level and decouple level from skill expertise. As for the thresholds, my version of Shadow World has basically two : 10 (beyond which you *need* to deal with the hidden world voluntarily on a regular basis) and 20 (beyond which you must strive to understand the deeper inner workings of the world and how to act on them to pursue your own ideals). For player characters and most "exceptional" NPCs, 10 is almost a default assumption, but 20 is a critical threshold - I know several PCs who would never get higher simply because they do not have the mindset for it.