It's that simple.
Scale is the ratio between model size and full size.
All these examples are for the same scale. The various model fraternities express scale in different ways. Model railroads are usually built to a letter scale, such as HO, where 3.5mm on the model represents 1 foot on the prototype. If the scale of the plan is different than the scale of the model, one must determine the ratio between the two. If the scale designations are mixed, that is, the plan is drawn to a letter scale and the model will be built to a fractional scale, some simple, preliminary calculations will have to be completed in order to set the dividers to the proper ratio.