Hi,
I must admit that I find it extremely difficult to estimate the altitude of passing aircraft. Obviously it gets tricky if they're at tens of thousands of feet, but for example, the Tornados that passed en route to a flypast the other day looked to be at considerably less than 1000ft; presumably they wouldn't have been lower than 500 over a built up area.
That said it's misleading, because you watch these things bouncing around the sky in a fairly agile manner, and then wander up to one on the ground and are immediately struck with the thought "Christ, this thing's absolutely enormous."
I presume people who fly regularly get much better at it.
Phil