Use the INNER diameter, not the name of the pipe. A pipe called fifteen is not fifteen millimetres inside, and because the radius is squared that difference counts double. On a long circuit it easily puts you a quarter off. The tool does not fill in inner sizes for you: they depend on the material and the wall thickness.
What goes in per hundred litres is printed on the label of the product you use. This tool only does the arithmetic; it does not pick a product and it does not decide a dose.
