Whist I like the idea of a calculator the example you showed is very odd. So a couple sharing a room only slightly bigger pay almost double the rent of a single person. What is the point of them sharing the room? they might as well have separate rooms.
Also I'm not sure what renting situations are like all around the world but it also depends on the responsibility of the flatmates. For example if someone decides to up and leave with no goodbye who has to pay for the rent? If the house gets damaged with no one to blame who has to pay? the list of these questions goes on.
There is never a good solution to this problem and that is why it is usually a case by case basis. I personally like the idea of one person choosing what a room is worth and then if you want to live there then you pay that amount.
RE: Fair rent with roomies?