Personally, in my games, the proficiency bonus = tier.
• Proficiency +1: "Level Zero", high school
• Proficiency +2: Levels 1-4: apprentice, college student
• Proficiency +3: Levels 5-8: adventurer, professional
• Proficiency +4: Levels 9-12: master, expert (1e "name levels")
• Proficiency +5: Levels 13-16: grandmaster, great, arch, leader, founder
• Proficiency +6: Levels 17-20: legend (Wish)
• Proficiency +7: Levels 21-24: epic
So for a table that wants to be "realistic", it makes sense to me, the game can stop at either level 8 (adventurer) or at level 12 (master).