The grid code I use to help measure and position elements, using prawn

stroke_grid.rb

require "prawn"

module Prawn
  module Graphics
    def stroke_grid(options = {})
      options = {
        :at => [0, 0],
        :height => bounds.height.to_i - (options[:at] || [0, 0])[1],
        :width => bounds.width.to_i - (options[:at] || [0, 0])[0],
        :step_length => 50,
        :negative_axes_length => 0,
        :color => "000000"
      }.merge(options)

      Prawn.verify_options([:at, :width, :height, :step_length,
                            :negative_axes_length, :color], options)

      save_graphics_state do
        fill_color(options[:color])
        stroke_color(options[:color])

        stroke_bounds

        fill_circle(options[:at], 1)

        (options[:step_length]..options[:width]).step(options[:step_length]) do |point|
          fill_circle([options[:at][0] + point, options[:at][1]], 1)
          draw_text(point, :at => [options[:at][0] + point - 5, options[:at][1] + 10], :size => 7)
        end

        (options[:step_length]..options[:height]).step(options[:step_length]) do |point|
          fill_circle([options[:at][0], options[:at][1] + point], 1)
          draw_text(point, :at => [options[:at][0] + 10, options[:at][1] + point - 2], :size => 7)
        end

        self.line_width = 0.5
        (0..options[:height]).step(5) do |point|
          tp = point % 50 == 0 ? 0.5 : 0.1
          transparent(tp) do
            stroke_horizontal_line(options[:at][0],
                                   options[:at][0] + options[:width], :at => options[:at][1] + point)
          end
        end

        (0..options[:width]).step(5) do |point|
          tp = point % 50 == 0 ? 0.5 : 0.1
          transparent(tp) do
            stroke_vertical_line(options[:at][1],
                                 options[:at][1] + options[:height], :at => options[:at][0] + point)
          end
        end
      end
    end
  end
end

Prawn::Document.generate("grid.pdf") do
  # Remove canvas to draw grid within current bounds
  canvas do
    stroke_grid
  end
end