Give the length one way; the tool counts the return conductor itself. A warm conductor has more resistance, so a cable working at seventy degrees drops more than the same cable at twenty. That difference is exactly what installations get failed on.
What this tool does not tell you
It does not say which conductor size you should install, and it does not say whether your voltage drop is acceptable. That depends on the standard in your country, on the type of circuit, on how the cable is laid, on the ambient temperature, on grouping and on the protective device. Look it up in the standard that applies to you and have the work inspected. A tool that gives you that number gives you a dangerous number.
