So, you're using standards-based grading.  Rather than recording quizzes and tests by name, you record numbers on some sort of sliding scale connected with a narrative to describe how each student is doing related to specific learning targets.  Next comes the hard part: 

How do you handle student re-assessments?
I've lost track now, but it seems like a few folks, including myself, have hashed out the pros and cons of in-class vs. outside of class re-assessments.  I teach high school kids.  They have ways (or can figure out ways) to get to school early and/or stay late.  Ideally, I'd like to handle them during class because I feel like anything outside of class is optional (or at least more optional than the stuff we do during class).  If it's optional, then is it really formative assessment?  Maybe it should be named summative-but-possibly-formative-if-you-take-advantage-of-it-assessment.  I don't know.

Here's what I've done in the past.
If a student wants to improve his/her learning target score, there's a bit of up-front work involved.  Student must come in outside of class for an extra help session or complete some problems outside of class to show me that he/she is serious.  I explain it like this to my students:
If you want to improve your learning target scores, guess what...I want you to improve, too!  Why would I NOT want you to learn?  I want everyone in this class to have all 4's in the grade book.  I want everyone to know everything there is to know about Geometry.  If you want to show me you can do better, I'll give you the chance to do so, BUT it's going to take some extra time on my end to create a new assessment, so I expect you to put in some extra time, too.  You can come in for an extra help session or complete some problems related to the learning target.
Most of the students relish the opportunity to....drum roll, more problems!  I don't want them to blindly re-take assessments every day, so completing some extra problems is a great way for them to think about the learning target and ask questions if necessary. 

The problem:
Coming up with problems for each learning target on a whim became cumbersome and difficult, so I started relying on the textbook.  Still, it was a paperwork mess to keep track of the problems for each learning target, collect student work once it was done and check it.  

The solution:
It took me a year to figure this out.  I wish I would've thought of it earlier.  I already post the answers to the homework, so I thought, why not post the answers to these remedial problems, too?  While I was at it, why not post the problem sets for each learning target so that students could help themselves, check their work and then turn in their work when they're done.  Students get quicker feedback.  It's less of a headache for me.  Winner, winner.  Chicken dinner.

So far it's working out rather well.  Yeah, students could copy each others' work.  It wouldn't get them anywhere though, because they'd still bomb the re-assessment due to their stagnant understanding of the learning targets.

This is my last year in the classroom, but I'm still interested in your thoughts as I help move the standards-based grading express forward in my district.  How do you handle re-assessments?