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Tableau V 2018.2 has a nifty feature in the dashboard editor called "show grid". If you're on that version, use that.
Before 2018.2 I would temporarily float a container and size it 3 pixels wide or so, and have it run from top to bottom, and then use that to align things. (Or 3 pixels high and run it from left to right. And/or multiple such containers.)