For those who mount the levers before putting the bar into the stem, there's an easy accurate way that works most of the time.
Mount the levers and tighten one. Place the bar on a table and, holding the levers out, roll it forward until the tips of the levers touch. You want 4 point contact, like a table that doesn't wobble.
This method doesn't work as well if the levers extend below the line from the bottom of the drops, but otherwise it's quick and easy.
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