How do you locate the stanines in an assessment?

I'm guessing a bit here, but how about the following:

 

Suppose your numbers are x₁, x₂ and x₃

 

Calculate the mean:  \(x_{av} = (x_1+x_2+x_3)/3\)

 

Calculate the standard deviation:   \(s = \sum_{k=1}^3(x_k-x_{av})^2/2\)

 

Calculate \(score_k = 2\frac{x_k-x_{av}}{s}+5\) for k = 1, 2 and 3

 

Then score will have a mean of 5 and a standard deviation of 2.  

 

Round the resulting values to the nearest integer to get the stanine values.

 

As I said above, I'm guessing here!