You have to forget the 2x, 3x, 4x zoom when it comes to interchangeable lens camera. The P&S cams have this because their lenses have a fized focal length for min and max zooms and you can then have a multiplier factor. But when changing lenses this cannot apply. For example my lowest focal length is 10mm on my 10-20mm lens and max is 500mm on my 150-500mm lens. So can I say I have a cam with 50X zoom?
Here 250mm is 250mm. The lens will give you reach from 55-250mm focal length. Call it 4.5x if you like. Or call it 28.8X since you will have a 18mm kit lens. I am not sure which of these if either will be correct.