How to add a rating scale with different weights

[HelpMePlease12]

Hi I am looking on how to add rating scales with different weights to calculate a final score. 

I am trying to create fillable PDFs for people to complete performance evaluations on when they can't access our online system. 

In this scenario I have 8 questions and the questions have different weights for the final score. 

Question 1-5 and 7 have a 15% weight

Question 6 and 8 have a 5% weight

both the employee and manager will give a rating for each question which then gets calculated to an overall score. 

[Lorraine22078102efxf]

Looking to do an average of up to 3 fields. I'm able to do determine the value for 3, however, if only 2 have a value, I want it to average for 2, not 3. What formula do I need to use? I think it's an IF THEN statement, where IF would be null; but that's all I've got. As you can tell, I'm not a java user

Thanks COMMUNITY!