The guideline is whatever works for you is right. Everybody is different, as is everybike. Differences in the relative height of the saddle and handlebar, top tube & stem extension, torso length, body angle, etc. all make trying to use a one-size-fits-all formula impractical. If it works, go with it. Later on you might tweak it a bit, and dial it in a bit better, but it's still a personal issue.

FWIW- while most shops set road bike bars up with the bottoms level, I've never been happy there. For 40+ years all my bikes have had the bars rotated forward a bit so the bottom slopes down at about 10 degrees. The brake levers are positioned a bit further down to compensate. That arrangement doesn't work well with anatomic bend bars, which is one reason I never liked them.