I'm working on a 15" x 21" poster than folds into a 5" x 7" card for mailing. The poster will be folded open, but the content will not necessarily be neatly contained in each 5" x 7" section — some content takes over two panels, some all three).
What is a best practice for creating a grid for such a poster?
Currently my grid divides each 5" x 7" section, with gutters that allow for room across each fold. While this ensures that small type near the fold will not cross it, the poster contains content that spans multiple columns and sections. This results in an uneven spacing of said columns (six columns appear more like three columns on one section followed by a large gutter and three more columns in another section).