The problem is that there really isn't a standard. And with the introduction of frames with sloping top tubes, seat tube measurements aren't that helpful anyway. Typically you see MTBs measured in inches from center of BB along seat tube to top of top tube and road bikes are measured in centimeters from center of BB long seat tube to center of top tube.

Some will measure frames with sloping top tubes from center of BB to a point that would be the top tube if the top tube was not sloping. Top tube length is typically a better indicator of fit than seat tube given the nature of frames nowadays.