max_height_from_desired_width.rb

class DoorCore
  def initialize(input_width)
    @min_height   = 2120.to_f
    @max_height   = 2455.to_f
    @min_width    = 915.to_f
    @max_width    = 1080.to_f
    @input_width  = input_width
  end

  def predicted_max_height
    @result = @max_height - (@input_width - @min_width) * (@max_height - @min_height) / (@max_width - @min_width)
    @result.floor
  end
end

puts "\n---------------------------\nWhat is your desired width?\n---------------------------\n"
desired_width = gets.to_f

app = DoorCore.new(desired_width)
output = app.predicted_max_height

puts "\nResult: #{output}"

and for calculated width

class DoorCore
  def initialize(input_width)
    @min_height   = 2120.to_f
    @max_height   = 2455.to_f
    @min_width    = 915.to_f
    @max_width    = 1080.to_f
    @input_width  = input_width
  end

  def predicted_max_width
    @result = @max_width - (@input_height - @min_height) * (@max_width - @min_width) / (@max_height - @min_height)
    @result.floor
  end
end

puts "\n---------------------------\nWhat is your desired width?\n---------------------------\n"
desired_width = gets.to_f

app = DoorCore.new(desired_width)
output = app.predicted_max_height

puts "\nResult: #{output}"