you have to design all 8 quantization levels for the give signal in order to make the signal-to-quantization noise ratio to be as large as you can. We have discussed in the class on how to determine the SNR.There are two major parameters that have to be determined;that is,the signal power and the quantization noise power.
You certainly need to iteratively choose the appropriate set of 8 quantization levels so using computer programming is a wise idea to help on doing these calculations.