Backgrounds comparison chart.
Level advantage just divides the average attribute modifier of each background over the apprentice baseline by the average amount of points you can increase that attribute on level up with no stars (2 for matk and mdef, 4 for ini, 3 for the rest), then divides that by 3 (since you can increase 3 attributes per level up).
So for example thief has a level advantage of 3, this means the attributes of a level 1 average thief are the same as those of a level 4 average apprentice. This doesn't take into account talents since those have different value depending on what attribute they affect and they are random so that's more of a brother-by-brother thing independent of background.