Are you doing buttercream? Could you do the red stripes in fondant? That might give you the cleanest sharpest contrast for the stripes. http://www.usflag.org/flagspecs.html will show you how to do the measurements if you really want to be accurate. Not too accurate though, I'm told you can't copy a flag exactly for any purpose other than a flag. Probably more info than you needed or wanted...
Maybe I'm not understanding but you could just use a ruler and pizza cutter and cut your stripes and lay them on the cake.
Hi Charmed, I just made a flag cake last night. This is an Idea if you are using fondant to cover your cake. I used a Fondant Ribbon Cutter. It makes perfect stripes. First I covered my cake in white fondant, then added red stripes going across and down the sides. I also used to Gum Arabic (Edible Glue) to adhere the stripes. then I pushed down the stripes a little to mold them to the white fondant.
Hope that helps. 
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