Grading System Recommendations

Personal Background:  I'm in my third year as a secondary math teacher.  This is a second career for me at 59yo.  Though I've been through course work, training, and testing, for my certification, I still lack experience especially when thinking outside the educational box.

Student/Class Background:  I have a class of Pre-Algebra 7th and 8th graders in which half are failing with very low percentages and the other half are 90% or better.  I have no kids in the middle.  I've divided the class into two groups.  The more remedial group I have primarily put on Khan Academy at a 4th/5th grade level to start catching them up.  I'm using "Assignments" with this group instead of "Mastery Goals."  It seems to be easier to track the students.

My Issue:  So I need some way to give them a daily or weekly grade.  I need to keep this relatively simple for me and fair to the students.  I now have added 10 preps to my workload.  My thoughts are to base this on their progression (to keep them on task) and the attempts (to prevent just guessing) they made to become "proficient" at a given skill.

Does anyone have any recommendations for a grading system?

What kind of expectations should I put on a student's progression in light of the number of attempts per skill for a week?

How many skills per week would be reasonable in a week?

How many attempts per skill would be reasonable to expect from an average student going through the given instruction?  (I'm thinking that I'll need to limit this to provide a better intervention.)

The students don't progress until they are proficient.  I also give each student an opportunity to "test out" of a unit before I assign any skills within that unit.

Thank you for any direction or recommendations!

Duane