The horizon is always at eye level with the observer, even on an airplane.
You would be hard pressed to find a less scientific and measurable statement. Have you tested this with any instrumentation or device with any level of accuracy?
Saw a reference to a theodolite app for the iPhone. Looks interesting. I'll ask Tom in advance of getting it, would measuring horizon angles with something like that be accepted by you as accurate? Would you expect it to always show the horizon at zero degrees elevation or something else?
On the topic of those theodolite apps read this metabunk thread.
I will have a read through this thread, but don't have time right now. Before I do, it looks like a general discussion about the "horizon rising to eye level" concept, that contains some reference to the apps like I mentioned (maybe the same app).
If you are offering this web page as reference, I assume you are prepared to accept all their conclusions. Is that correct?