Grading growth

I teach a class that was created as an intervention class for students with low ACT scores. Our school has recently become part of a blended learning pilot program, meaning that I have a class set of laptops now. In discussion with my principal, we have decided to make this class much more individualized, since one of the difficulties in managing that class is the huge gap between the highest and the lowest students. I am giving them a diagnostic ACT test and trying to figure out then where each of them should start their remediation. In order to run the class this way, we have to figure out a fair way to grade them based on their growth and effort. I’m thinking a rubric may be the fairest way to do this, but am stuck on what my categories/indicators would be. Got any ideas or even guiding questions for me?