According to a book i have been reading, inkjet matte/luster/glossy paper recommended output settings are all at 240ppi or 360ppi, with inkjet glossy also at 480ppi. the book is "The creative Digital Darkroom" and they primarily reference Photoshop CS3. maybe check with the magazine if 300 ppi is their minimum resolution for print. Of course, they are probably not using an inkjet printer but you want your photos to really look good in that magazine.