It would be easiest to calculate it by using infrared underfloor heating calculator.However, if you are calculating manually, then take only 90% of the whole area (also, do not count the space under the stationary furniture).
Then, place infrared underfloor heating film accordingly to your room width. For example, if your room width is 8 meters, place 8 stripes of film of 1m width OR 10 stripes of film of 80cm width, and etc.