I have several units that I have purchased over the years, and one that I custom made and mounted to my drafting table. The secret to using them is starting with a secure mounting point. They do take up a considerable amount of room to operate depending on the size difference of what you are transfering. The ratio is scaled on the pantograph itself and should be easy to figure out once you use it. My question is, what do you want to do with it ??? These can usualy be found at any store that offers art or drafting supplys. If you want to make your own, you can make it to any scale or ratio you want. Jim S