How to find the best values for contour function?

I have to extract the nuclei from cervical cells images and I have to use the contour function in MATLAB. But I don't understand how to define the vector of values, because some nuclei don't get selected well. For example for this image (1), using this vector [130 170 30 70] I got this result (2), but I just used some random numbers that seemed to me to represent better the nuclei. How should I select the values to be sure that it is the best solution? Thank you and sorry for my bad English if I didn't express myself correctly. PS: the image that I work is converted into gray levels.