Some points to keep in mind when you requesting a recommendation letter:

1) if you took one class from me and got a B, the letter is unlikely to be helpful
to you (unless it is a B in a graduate class).

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.
So I must disclose to the graduate programs the detailed information about the
meaning of the grade. 

In particular, 
I am expected to disclose your percentile ranking in the class and describe
the level of hardness of the class. So, being in the upper 25% in Calc 3 
certainly won't hurt but all alone I don't think it is going to get you 
into an Ivy League school. 

If you are unsure about your standing in a specific class, you can always ask me for 
this information. This is a hard fact, not an opinion.

3) Unless somehow I got to know you personally, I will not be able to write anything 
else besides this ranking information.