SBG Grading Scale Confusion

I am implementing standards based grading (SBG) in my first year of teaching. I was encouraged by the great deal of reading I have done in the MTBoS on the subject, as well as by the experiences of my uncle and another peer who use it. I have been bothered by one aspect in particular - how to convert my 0-4 scale into our school's grading system, which requires a percentage input. Someone I know whose school uses the same grading system as mine enters the 0-1-2-3-4 score into the system as a number out of 4. So if a student gets a 3 on an assessment, their grade is 3 out of 4 = 75%. That sounded really simple to me, so I did the same. Until my advanced class started questioning me on my scale, and I got a phone call from a parent about the same thing. I stuck to my guns because SBG is something I believe in - if only from the conversations I have had or overheard about it - and I wanted to give it time to see if the problems would work themselves out. However, it brought to the forefront all of the concerns I was having about the way I converted my score to a grade in the gradebook, and made me prioritize the issue. I have only a few connections who I can turn to for questions about SBG, and they recommended converting the 0-1-2-3-4 scale into a letter grade scale. So a 4 is an A, a 3 is a B or B+, a 2 is a C, a 1 is a D, and a 0 is an F. When I look at the criteria in my rubric (which I stole shamelessly from Dan Bowdoin) it makes much more sense that a 3 is a B or B+ rather than 75%, which is a C. So now I have the work of notifying parents and students of the change, as well as changing every grade I have in my gradebook to reflect this new conversion.

Thinking about this grade conversion made me realize how many grading scales we use in U.S. schools, aside from the SBG scale (which is variable; it's not always a 0-4 scale like mine). We have letter grades, percentages, and GPA. How did it come about that we have so many different scales? I wonder if I'm only adding to the mess by creating another scale. However, I believe the SBG scale has the potential to get students to focus on how well they know a concept, rather than the number of questions they get right. It's supposed to focus their attention on mastery, rather than grades. Using SBG is totally new for me, and I have a lot of things to work out. I hope this is a step in the right direction.

Score	Number in Harmony	Letter Grade	Criteria
4	100	A+	Complete understanding of the concept with no mistakes.
3.5	94	A	Complete understanding of the concept with a few small errors.
3	88	B+	Strong understanding of the concept but making mistakes.
2	75	C	Basic understanding of the concept but needing more practice.
1	65	D	Very little understanding of the concept.
0	55	F	No understanding of the concept, or no work shown.