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You can use Custom scripts and type in-the code along the following lines:

 

function stretch(val, min, max)  {
  return (val-min)/(max-min);
}
return [
  stretch(B04, 0.05, 0.5),
  stretch(B03, 0.05, 0.5),
  stretch(B02, 0.05, 0.5)]