Calculate "Overall Rating" based on Other Field Value Pairs: Rating and Weight

I have found plenty of tutorials on how to set a read-only field's value as calculated off other field values entered into the form. However, I have a situation where our HR department is needing an evaluation form to present the manager 10 different ratings areas for various aspects of an employee's job responsibilities but they are also allowed to set the weight of the value for calculating an overall value from these 10 areas of rating. 

Brief example: 
Responsibility 1 = 5
Weight R1 = 25%

Responsibility 2 = 3
Weight R2 = 10%

Responsibility 3 = 5
Weight R3 = 65%


What I need to do is find a means of calculating the "Overall Rating" based on the Responsibility value and weight given for how much of the overall rating should be impacted by said value. In my example above I have 3 areas rated with a total weight of 100%. Is there a means to leverage these values to calculate the "Overall Rating"? Can the solution be such that if the manager only uses 7 of the 10 areas, that the calculation will only be determined on the 7 rather than including the 3 they omitted? 

Any and all help would be appreicated in this matter.