Another possible slight variation on that could be proficiency -2:
1st : +2/+0, at 5th: +3/+1, at 9th: +4/+2, at 13th: +5/+3, and at 17th: +6/+4
This one tackles the high level splitting off of the proficient vs. not proficient better than just 1/2 proficiency. The two numbers do split still...