Not sure if I understand the problem correctly, let me know if I am wrong.

you have an area that will be in use for 20 hours a day, regardelss of how many people are there, so you effectively want project to work out nmumber of people if you tell it how many hours were worked?

I would suggest that you change the task type from Fixed Work (which I assume it is on) to Fixed Duration. this would enable you to inout the amount of work done, and project would maintain the task as a 1 day duration.

Again sorry if this isnt what you are asking still a tad confused with it.