Here's a brief explanation of how I estimate your overall grade.  This will also give you some idea of how PackBack participation affects your overall grade.  This might be a little confusing, so proceed at your own risk.

Let's assume the following scores



The first estimate doesn't drop any quiz scores.



I try to estimate the grade you will have at the end of the semester if you keep on doing as you have been doing.  Because of this I have to guess at what your Quiz #3 and Quiz #4 scores.  I assume they will be comparable to your Quiz #1 and Quiz #2 scores.  The 80% value above is the average of 4 quiz scores plus the writing percentage grade. 

If you are not part of the PackBack online discussion I would add the 0.9 pts of extra credit and end up with an overall average of 80.9%.

PackBack participation at the 5% level (you just read other student's questions and answers but don't contribute any of your own) would raise your overall average to 81.9%.  It has a small but beneficial effect.

Neither of these overall grades is 90.0% or above, so this student will not be exempt from the Final Exam.  I compute a second overall grade after dropping the lowest quiz score. 



The 72% is dropped, the 82% Quiz #2 score is left.  I assume that the Quiz #3 and Quiz #4 scores will be the same (this might not be very realistic).  The average of the three quiz scores and the writing percentage grade is 84.5%.  Without PackBack the 0.9 pts of extra credit raise this to 85.4%, a solid B.  Including the PackBack score raises this a little bit to 86.2%.