You may have to adjust how you fold the 'zines if you are not printing on 8.5" x 11" paper (like, people across the pond often print A4) or if your particular printer cannot print as close to the margins as expected.
I believe if you're printing centered on A4 and there aren't any margin-rescaling issues, you'll trim 6.25cm off of each end, leaving a center of 27.2 cm or so. But you may have to adjust based on your printer.
Ideally, there should be roughly equal margins on all sides of each "page", which will be determined by the interior margins between pages. Basically, you'll trim the sides of the page until the margins on the "outside" of the content matches up with the margins on the "inside" of the content. You can measure it with a ruler if need be, or you can "eyeball it" by doing some of the folds and hand-cutting matching margins on the outside of the folds.
(Another approach is to just fold the parts of the 'zine until the pages look right and then trim.)