1) If you took one class from me and got a B or B+, the letter is unlikely to be helpful to you (unless it is a B in a graduate class). Most likely, I will decline to write the letter, although I did once write a letter for a student who got a C. There were some unusual circumstances in that case.

2) if you got an A or A-, you should still keep in mind that the essence of a good recommendation letter is a concrete comparison with other students. I am expected to disclose your percentile ranking in the class and describe the level of hardness of the class. Being in the upper 25% in Calc 3 certainly won't hurt you but all alone it is not going to get you into an Ivy League school. If you are unsure about your standing in a class, you are welcome to ask me for this information.

3) Unless I got to know you personally, I will not be able to write much besides this basic ranking information. If you want me to comment on something you achieved, tell me about it.

4) Remind me about deadlines.