Quantization levels of received symbol for viterbi decoder

i am designing viterbi decoder. i have one problem. i am doing 3bit quantiser for quantizing the received symbols(noise addedsymbols). but my problem is from which number to which number i have to take as quantization level. either from 0 to 7 or -3 to 4. depending on those the branch metric is varying. which will give minimum metric as best metric. i am planning to calculate hamming distance. if any body have the idea on this please mail me. thank u. bye