Why not?

If you can measure effective top tube length then you know were to measure to for effective seat tube length. While most bikes in the old days were measured from the BB center to the actual top of the seat tube some were also measured to the intersection with the top of the top tube and some to the centerline of the top tube.

And if you read a little further down on the page you linked then you'll see they list effective seat tube length. And take a look at the last part of that! I'd copy paste it, but their servers are down right now.