Calculating Light Fall-Off From Large Source

"the inverse square rule is often still a useful approximation; when the size of the light source is less than one-fifth of the distance to the subject, the calculation error is less than 1%"

When calculating light-off from a source substantially larger than normal (6m x 1m, 10m x 5m) with the subject being 1m – 10m away from said source how would one approximate calculating format in a practical sense. Assuming all portions of the diffuser used is uniform I’ve read "then the total illumination on your point is proportional to the solid angle from the point that intercepts the light".

If, for example, I had a source 6 metres in width & 1.5 metres in depth. With my subject being approximately 10 metres away with total output being 6600 FC, 1′ away from the source or 71041.81 LUX, 30.5cm away from the source. How would one put that into a practical equation.

Thanks Gabriel