I am building an LED-light and am trying to figure out how many LEDs I need to be on it. I know how much illuminance I need as well as the distance to the subject and the luminous flux of one of the LEDs. The problem is that the LED's data sheet doesn't provide a clear viewing angle but instead a directivity diagram / radiation pattern (page 11).
Question: How do I compute the illuminance of one LED (26lm) at a given distance (1m) based on the directivity diagram / radiation pattern?
Edit: More specifically, I would like be able to compute the average illuminance on a surface of a given size at a given distance, for example the average illuminance on a circle with a radius of 10cm at a distance of 1m that's center point is straight in front of the LED and that's not tilted in any direction.