You can calculate the drop in the globe model via geometry. Gravity is a theory. There are no proofs for it only hypothesis. You can explain everything with buoyancy and electromagnetism. There is a lot of material which describes this in detail better than I can.
Discussion
I have come across the buoyancy theory before and at face value I suppose it makes as much sense as gravity, but I haven't researched it in depth.
I'm curious though, assuming it's true that the earth is flat and we are inside a firmament, how do you feel this affects your daily life? Is there an actionable change in the way you are living as a result of knowing this?