Teachers are under no obligation to offer any extra credit at any time.  We do it to allow some wiggle room for students on the cusp between a good grade and a great grade, or a failing grade and a passing grade.  If you fail to achieve the grade for which you aspire after 15 points of extra credit, you don't deserve a higher grade.  

Your December 2018 semester tests were calculated in the following manner.

 Step 1:  (Test score  / Total points possible) x 100 =  "x" Points (now on a 100 point scale)

 Step 2:  Extra Credit: 

A:  5 points max for XC on CCI (Scoring greater than 5 points on the CCI will still result in an award of 5 points)

B:  5 points max for XC on daily work, and lab/graph/presentations where teacher is absent based on substitute scores. (Scoring greater than 5 points will still result in an award of 5 points)

C:  5 points  test curve (based on top test score)

     Maximum  Extra Credit = 15 points

 Step 3:  "x" points + max extra credit = semester test score 

Example:

Student scores 44 points on a 77 point test

Step 1:  (44/77) x 100 = 57 points (now on a 100 point scale)

Step 2: Part A: +5

Part B: +5

Part C: +5

           Total XC =  15 

Step 3: 57 + 15 = 72 pts

Benefit: Without the extra credit, student semester test score would be a 57%(F).  With maximum extra credit applied, student score is a 72 (C-).