The Howgozit only "works" if:

You don't change ANYTHING.

Fly faster to make A:00 - will be off.
Fly .80 in a 764 instead of the ridiculously slow .765 that SABRE filed you at - it will be off.
Intentionally fly slower to make a curfew landing time in LHR - it will be off.
Fly different altitude for smoother ride - will be off.
Get a track change - will be off.

The howgozit does NOT update anything that you've changed from the SABRE flight plan.

So, it's a "ballpark" comparison of what you're doing vs what you were planned to do.

Do I ever look at it once I print it - no.
In fact I ask if guys even want it printed - some say yes, most say don't bother.

But I do leave the progress page up on the FMC to make sure the fuel stays close to what it said when we took off.