So here's the breakdown:
My current Chemistry grade is a C+. If I get an 87.4% or higher on the final, I can bring it up to a B-. If I get a 71.4% or higher, I can get a C+, and as long as I don't get any lower than a 47.4%, I will get a C.
My current Calculus grade is a B. There is no possibility of me getting an A- or a B+. I need an 83% for a B and need to get above a 68% to get a B-. I wish this final were optional. I also wish grades were not based solely on exam scores without calculating lecture attendance or homework like my other classes do.
My current Physics grade is a B. I need a 91% on the final for an A- and an 82% for a B+. I need to get above a 70% in order for the optional final not to hurt my grade.
I have an A- in Psychology, and taking the final cannot increase my grade, so I'm not taking it.

All of these grades are before any calculated curve.