i suggested that because the 'horizon' is ever changing with hardware, so you can't possibly keep up. but all tablets are 'generally' the same size, and all phones are 'generally' the same size, in relation. meaning, your layout will look pretty much the same on all tablets if you create a layout for tablets, and likewise for phones. so it might be easier to just a comparison cut off; from x pixels and up - use the tablet layout, and from x pixels and down, use the phone layout.
but of course, you're free to take your own route. i'm just explaining this method in detail for others who may want to use it.