Searchkick reindex with resume

searchkick_reindex_resume.rb

namespace :searchkick do
  desc "maybe reindex without running out of memory?"
  task :altindex, [:start_index, :batch_size, :new_index_name] => :environment do |t, args|
    ActiveRecord::Base.logger = Logger.new(STDOUT)
    count = Item.count
    start = args[:start_index].to_i || 0
    batch_size = args[:batch_size].to_i || 1000

    # we create a new index or take one already present
    if args[:new_index_name]
      new_index = Searchkick::Index.new(args[:new_index_name])
      puts "we took a existing index"
    else
      new_index = Item.searchkick_index.create_index
      puts "the new created index name is: #{new_index.name}"
    end

    puts "Starting reindex at #{start} with batch size of #{batch_size}..."
    Item.find_in_batches(:start=> start, :batch_size=> batch_size).with_index do |batch, index|
      puts "Processing batch ##{index}; total #{index*batch_size+batch_size} items"
      before = Time.now
      new_index.import(batch)
      time = Time.now - before
      puts "Processed #{batch_size} items in #{time} secs"
    end
    Item.searchkick_index.swap(new_index.name)
  end
end

How do we know where to start? or how did you get the start value?