Originally Posted by
Kontact
That's "Head Tube Length" on geometry charts, and it is a slightly elusive concept now that headsets can be internal or external and doesn't reflect on fork length. For what you are asking, "Stack" on geo charts is more useful because it is the height of the top of the head tube above the BB.
19.5 might be low, or it might not include some things important for comparison. I don't know what Planet X bike you have.
Head tubes may have gotten marginally shorter as fork blades got marginally longer, to accommodate modern fat tires. On the other hand, that space may have simply been taken from where the crown accommodated the brake bolt. And regardless, we're only talking about a few millimeters. But measuring the two bikes as I outlined in my most previous post should show you where it went sideways.