Originally Posted by Dave Cutter

You should be able to stand flat footed over the top tube. Meaning the highest "bar" should be comfortably under your body if you have to jump down at anytime during the ride.

I remember when I was a child (decades ago) there was an idea that we should be able to remain seated at a stop sign (or light) yet keep the bike upright while on our tippy-toes. I don't know where that came from.... but that "guide" isn't used now-a-days.