Your Menelik number is calculated by dividing your age at your closest birthday (ie, if you turn 20 in three months, put 20, not 19) by the number of countries you've visited.
For example, John is 21 and has been to Sweden, Finland, Germany, Thailand, Indonesia and New Zealand, as well as his native Australia. He has a Menelik number of 3 (21/7). Ann is 34 and has visited Mexico and Jamaica; she's from the USA. She has an MN of 11.67 (35/3), because her birthday is in 6 weeks. Gordon is 12 and has been to Ireland, France, Spain, Portugal, the USA, Canada, Belgium, Luxembourg, Italy, the Vatican City, the Netherlands and Egypt; he's British. His MN is 0.92 (12/13)
Mine is either 3.29 or 3.83, depending on what you count