Use variable in SPICE parameter list
All parameters need curly braces around them, this lets the compiler know that it's a parameter. Another tip is to check the netlist by going to view->spice netlist to see what spice code the graphical interface is 'compiling' 


  The .param directive allows the creation of user-defined variables.
  This is useful for associating a name with a value for the sake of
  clarity and parameterizing subcircuits so that abstract circuits can
  be saved in libraries.
  
  The .param statement can be included inside a subcircuit definition to
  limit the scope the parameter value to that subcircuit.
  
  To invoke parameter substitution and expression evaluation, enclose
  the expression in curly braces. The enclosed expression will be
  replaced with the floating-point value.
  Source: http://ltwiki.org/index.php?title=PARAM_User_defined_parameters