Exact SNR Calculation for ML receiver in Simulink

Hello .. I have a major problem when trying to calculate the Exact SNR
from the received symbols. Usually, in Simulink it is pretty simple to
calculate the SNR per symbol by calculating the variance of the
difference between transmitted and received symbols. This works perfect
for the ZF receiver. However, for the ML receiver, this method is not
possible any more bcuz the ML receiver maps the codeword to the nearest
codeword and then the variance is LOST. the output of the ML receiver
is a codeword of either +1 or -1 assuming BPSK for instance. Howeverr,
for the ZF, the output was any value between -1.5 and 1.5. For example
0.8, -0.95 which allows some sort of variance.
I would really appreciate if someone could propose an intutive solution
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