1) Be Arbitrary
There's no way around it, when trying to convert standards-based grades to a 0-100 scale there is no good way to make it work. At some point you will have to make an arbitrary decision about how you convert to a traditional grade. Rather than lose sleep over how you are going to convert, just acknowledge that there is no good way to go about it and make a decision you can live with. This may actually help you win more support to transition to a standards-based system, but more on that in a minute.
If your school uses a 10-point range for each letter grade, here is how I would convert a 4-point proficiency grade to a 100-point scale :
4-100, 3-95, 2-75, 1-60
In this case, a student who exceeds the standard (4) would get the highest grade possible, but you will have a hard time giving a student who did everything they were expected to do and "meeting the standard" (3) anything less than an "A" unless your school typically award "A's" to only outstanding work. The lower scores are harder to convert and probably depend more on your school's grading philosophy. A "C" makes sense to me for a "2", but I could also see this being lower (notice that we completely leave out the option of a "B" here). If a "1" represents a student who did the work but it did not even partially meet the standard, a grade around the failing mark makes sense, but you may also just wish to call it "incomplete." The old adage, "A, B, and 'you're not done'" fits nicely with the standards-based philosophy.
2) Be Clear
You're going to need to be clear in a couple of ways. First, you need to make sure your students know what standards they are working on as they work in class and take assessments. By being explicit about this you will help them start to associate their score with becoming proficient and not with some other measure.
You will also want to be clear in your communication to students and their parents about what your system is and how it translates to their grade in what we often call "the old world." This is the time where you can win over public support, because if your parents understand what you are doing and see the good it does for their children, they will help to push your school out of that old world of letter grades and in to a standards-based system.
3) Have An Alternative Reporting System
If you are one of the first people in your school to make the move to standards-based grades, odds are you school will not yet have a system in place to report out student progress (and when they do, it will always be ready for you and parents "next month"). This means that you will need to come up with an alternative system in place of or in augmentation to a traditional report card.
This system could be as simple as logs students keep themselves to online spreadsheets that can be shared with each individual student and their guardians so everyone has real time access to their progress. Regardless, there are two things you should make sure to do. First, keep your labels simple enough that everyone can understand me but detailed enough so everyone knows what the student is showing proficiency at. Second, make sure there is a space for narrative feedback, as this will always be more meaningful than a number and will help alleviate confusion.
Those are my tips, what else do you think is key when converting standards-based grades in a traditional system?