When a test is worth, let’s say, 82 points, how can I assign a grade?

Divide the student score by the total score, then turn the decimal into a percent.

Suppose a student scores 75 out of 82 on the test. Think of it first as a fraction expressed as 75/82. 75 divided by 82 is 0.9146… which is 91%. The grading scale at most schools for 90%-100% is an A.