I set the bike up in a trainer with the hoods just loose enough that they can be moved around by hand. move them around while pedaling on the trainer until you find the position you like. tighten one of the levers. I use a flexible ruler to measure from the bottom of the lever body, around the radius of the bar to the end of the bar. note the distance and duplicate it on the other side. that seems to be what you're going for.
you can also get them about where you want them, then check them for even-ness by laying a long, rigid item like a level or a yardstick on top of the hoods. eyeball the tool to see if it's parallel with the top of the bar. adjust until the tool is parallel with the top of the bar. I find the former method to be more precise.