How to restart current_level with 3 missed_days?

When a user clicks all three of the checkboxes in a row (current_level):

How can we restart that level so that, for example, if they are on Level 3 and were on day 40 they are pushed back to day 26 (25 if you include the 0):

  case n_days     
      when 0..9     # Would go back to 0
        1
      when 10..24   # Back to 10
        2
      when 25..44   # Back to 25
        3
      when 45..69   # Back to 45
        4
      when 70..99   # Back to 70
        5
      else
        "Mastery"
    end

The _form (as represented by the above picture):

<label id="<%= @habit.id %>" class="habit-id"> Missed: </label>
  <% @habit.levels.each_with_index do |level, index| %>
    <p>
      <label id="<%= level.id %>" class="level-id"> Level <%= index + 1 %>: </label>
      <%= check_box_tag nil, true, level.missed_days > 0, {class: "habit-check"} %>
      <%= check_box_tag nil, true, level.missed_days > 1, {class: "habit-check"} %>
      <%= check_box_tag nil, true, level.missed_days > 2, {class: "habit-check"} %>
    </p>
  <% end %>

Then upon the level restarting, 3 additional checkboxes would have to show in the level row, in case the user misses days again.

MORE CODE

Full habit model

class Habit < ActiveRecord::Base
    belongs_to :user
    has_many :comments, as: :commentable
    has_many :levels
    serialize :committed, Array
    validates :date_started, presence: true
    before_save :current_level
    acts_as_taggable
    scope :private_submit, -> { where(private_submit: true) }
    scope :public_submit, -> { where(private_submit: false) }

attr_accessor :missed_one, :missed_two, :missed_three

    def save_with_current_level
        self.levels.build
        self.levels.build
        self.levels.build
        self.levels.build
        self.levels.build
        self.save
    end

    def self.committed_for_today
    today_name = Date::DAYNAMES[Date.today.wday].downcase
    ids = all.select { |h| h.committed.include? today_name }.map(&:id)
    where(id: ids)
  end 

    def current_level
            return 0 unless date_started
            committed_wdays = committed.map { |day| Date::DAYNAMES.index(day.titleize) }
            n_days = ((date_started.to_date)..Date.today).count { |date| committed_wdays.include? date.wday } - self.missed_days

      case n_days     
          when 0..9
            1
          when 10..24
            2
          when 25..44
            3
          when 45..69
            4
          when 70..99
            5
          else
            "Mastery"
        end
    end
end

days_missed_controller.rb

class DaysMissedController < ApplicationController
  before_action :logged_in_user, only: [:create, :destroy]

  def create
    habit = Habit.find(params[:habit_id])
    habit.missed_days = habit.missed_days + 1
    habit.save!
    level = habit.levels.find(params[:level_id])
    level.missed_days = level.missed_days + 1
    level.save!
    head :ok # this returns an empty response with a 200 success status code
  end

  def destroy
    habit = Habit.find(params[:habit_id])
    habit.missed_days = habit.missed_days - 1
    habit.save
    level = habit.levels.find(params[:level_id])
    level.missed_days = level.missed_days - 1
    level.save!
    head :ok # this returns an empty response with a 200 success status code
  end
end

Here's the gist of it: http://ift.tt/1PISTF0

I'd be so grateful for your help here. I'm a beginner whose been grappling with versions of this problem for months! Thank you :)