Tontogary do you mean the magnetic field "flipping" when you refer to the dip?
https://news.nationalgeographic.com/news/2004/09/0927_040927_field_flip.html
It makes sense (to me) that the actions of the rotating earth would generate the heat and movement required to create the magnetic field and then for it to move over time. As I have never seen a Flat Earther state that the flat earth is rotating (only eternally accelerating in ONE direction) I can't see how this would cause the field to alter in the way it is observed (i.e. by the iron filings from volcanic eruptions being found to all point to where the north point was at the time of the eruption).
Devils Advocate, no, i am referring to the angle that the lines of force make with the horizontal.
As the earth is round, and the earth has a basic bar magnet internally, the lines of magnetic flux (force) come out of (and into) the end of the magnet at 90 degrees, so at the magnetic poles the force is 90 degrees to the earths surface. This makes standard magnetic compasses difficult to stabilise at high magnetic latitudes.
This is measured by essentially having a very finely balanced needle, pivoted in the middle, and seeing the angle that the needle takes relative to the horizontal. A bit like an old fashioned weigh scale, with the bar being the magnetic needle.
At the magnetic equator (approx near the geographic equator) the lines of flux are parallel to the surface of the earth, so there is no vertical force, only horizontal force.
As a very rough rule of thumb, at 45 degrees magnetic latitude, the dip is 45 degrees to the horizontal. At 30 degrees magnetic lattitude it is 30 degrees, and so on.
As part of my Master Mariners licence we study magnetism, and the different effects, and effects on a ship as we are required to demonstrate how to calculate different coefficients (forces caused by induced or permanent magnetism) in the 3 dimensions, vertical, longitudinal and athwardships directions, and correct a ships compass to overcome the coefficients.