Substandard deviation
MATL, score 65.30697
tYmhZs

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How it works
The built-in function Zs with its default arity (1 input, 1 output) computes the corrected standard deviation:
\$\sqrt{\frac 1 {n-1}\sum (x_i-\bar x)^2}\$
The uncorrected standard deviation can be obtained with the 2-input version of Zs: 1&amp;Zs, where 1 as second input means uncorrected. l or T could be used instead of 1 to reduce the score, but &amp; is very far from the other characters. 2$ or H$ could be used instead of &amp;, but $ is even farther.
So it is better to use the default version of Zs (corrected standard deviation) on the input with its mean appended. This increases the input length by 1 and contributes 0 in the numerator, which causes the corrected standard deviation to become uncorrected.
t     % Implicit input: numeric vector. Duplicate
Ym    % Mean
h     % Concatenate the input vector with its mean
Zs    % Corrected standard deviation