SImply a set of 9 Thumbnail frames that have a Rule of Thirds grid properly scaled to the frame.
The Canvas size I used was 3000 x 4500 pixels (10" x 15") at 300 dpi.
The thumbnail frames are scaled to the canvas' size, and should give you a good idea for how your thumbnail will scale up when you move onto further devleoping it for the canvas' full scale.
When setting up your canvas set it to 3000 x 4500 Pixels (10" x 15" inches) at 300 dpi, as thats what it's scaled for.
The bordering as well as the grid lines can be hidden as well.
Hope it speeds along some of your work.
Hope it speeds along some of your work.