Effective top tube vs top tube

What is the difference in the procedure to measure effective top tube length as opposed to top tube length?

Now that many bikes have slanted top tubes, or no top tube at all, a measurement called “Effective Top Tube Length” is often used. This is the measurement of what the flat (Horizontal) top tube on a bike would be if it were there. To measure the top tube length of a bike, the bike usually has an actual flat top tube and you just use a good old tape measure and get the length of the tube. To measure the effective top tube length (again the bike would not have a flat top tube) you would need to measure from the head tube to the seat post where the flat top tube would be if the bike had one.