calculate contrast color in Ruby

contrasting_text_color.rb

  def convert_to_brightness_value(background_hex_color)
    (background_hex_color.scan(/../).map {|color| color.hex}).sum
  end

  def contrasting_text_color(background_hex_color)
    convert_to_brightness_value(background_hex_color) > 382.5 ? '#000' : '#fff'
  end

how is the 382.5 benchmark decided ? White has brightness value as 0, and for black, it comes as 510, so why not 510/2 = 255 ?

FF FF FF is 255 255 255.

382.5 is (255*3) / 2