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You could use function objects to implement

Parameter{ 
Name    = normalizedSpacing;
Min     = 0;
Ini     =  0.5;
Max     =  1;
Step    =  0.1;
}
Parameter{
Name    =  radius;
Min     =  -3;
Ini     =  0;
Max     =  3;
Step    =  0.5;
}
Function{
  Name = spacing;
  Function = add(0.01, multiply( %normalizedSpacing%, subtract(multiply(2, %radius%), 0.01)));
}