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I found a solution for a 12 column grid , I was trying to create a 1200px grid and I had a hard time to make this right , but I may have found a solution. I set my the 12 columns with 12px gutter width and 89px column width and the margins to 360 , this works also with 1080px / 12 columns grid as well you just have to use 12x gutter width.
Hope this helps. Good luck.