Is it appropriate to ask a professor how my grade was calculated?

You can certainly ask the professor to justify how a grade was calculated and assigned. In most universities and most systems the professor is obliged by the rules to justify to students how a grade was obtained. Moreover, minimal decent teaching practice is that the manner in which grades are to be calculated and the conditions in which the evaluation is to occur are fixed in advance and known before evaluations occur and that the calculations are reproducible by the student evaluated (there are of course particular contexts, e.g. evaluating a student with discapacities or an exam that contains an error in a problem formulation or an interruption (e.g. power outage) in its administration, in which particular modifications might have to be made).

Assigning grades according to some scheme known only to the professor is considered something bordering malpractice in many countries. In Spain, where I work, a student always has a formal right to see how exams etc. were graded and to formally protest any perceived error in their grading.

In my opinion: If you have received your grade you can ask the Professor to provide some explanation. At least for us in Europe it is ok to ask. It just depends on the way you ask. Depending on the person he/she will always be offend when you ask. However, just asking for the individual points/grades and how your overall grade was calculated shouldn't be offensive. Why I think so: it should be in your own academic interest to know where you did something wrong and how you could improve.