AGL is always the altitude above the ground (directly below the aircraft) and has no relation to your launch point unless you fly on a perfectly flat prairie.
You don't have a way to measure this and neither does the pilot of a Cessna but that doesn't matter.
You can estimate it like the Cessna pilot has to.
The FAA is never going to be there with a tape measure and ladder to tell if you are flying at 405 ft or 395 ft.
That sort of detail isn't important.
Whether you fly in a reckless manner and endanger other users of the national airspace is.
^^^^^ Nailed it!!
In aviation we are often giving it our best "Estimate" and erring on the side of caution. The only time I know when my altimeter is exactly correct is when my wheels are sitting on the ground.