How to calculate inverse square law?

The original input is 70mW/cm2 for a light source.

I am trying to get to 10mW/cm2 for 10 minutes outputting about 6 J/cm2.

I understand each time I double the measurement, I quadruple the area covered so the power is 4x less...

But at what point do I start at, i.e. if I start at 0 and double the distance it won't compute. If I start at 1/2 inch the difference will be huge to if I start with 1 inch....

We're talking LED, 633nm and 830nm (but not together)

Thanks