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You want to scale from one linear range to another.  That can always be done with

     y = mx + b

where X is the input value and Y the output value.  M is the scale factor, and B the offset.

You want to map (-1 .. 1) to (0 .. 319).  The scale factor therefore has to be 319/2 = 159.5 = m.

Now that you know M, pick any known X and Y pair and solve for B:

     b = y - mx

You know that -1 maps to 0, so

     y - mx = 0 - (159.5)(-1) = 159.5 = b

The final result is therefore

     y = 159.5x + 159.5

Note that this method works for mapping any linear range to any other linear range.