How to get the size of a grid box

[Wagner Matos]

I'm trying to use grids to create ads within a page. Lets say I have a page with 5 columns and 5 rows.

I'd like to dynamically calculate the size of each subsequent box within the grid so I can - for instance - use an image to fit the whole width of the box. I've tried the following:

Prawn::Document.generate("test.pdf") do |pdf|
  pdf.define_grid(:columns => 5, :rows => 5, :gutter => 10)
  pdf.grid(7, 1).bounding_box do
    pdf.stroke_bounds
    pdf.text "Some text"
    pdf.image open("image_url"), :fit => [dynamic_size, dynamic_size]
  end
end

Ideally I'd like the ability to use percentage in the

:fit =>[100%, 30%]

parameters (or a method to calculate this percentage on the fly). 

Any ideas how can I achieve this?

Thanks in advance!