Its the "what" being doubled that might be causing the confusion. The addition amount isn't double, it's the amount in the container after the addition is doubled.
So I start with 5. My first addition isn't doubled, it starts the process. At the end of the second addition we need 10, so we add 5. Now we have 10. To double we add 10 and after the third addition we will have 20.
( to continue on to show progression of the factor for N)
The number of steps are 3, and we need a total of 10 lbs of rice to be used by the end of the third addition. So take 10 / (2^3 - 2^2) = 2.5, N = 2.5
N = TotalAmount / (2^MaxSteps - 2^(MaxSteps-1))
Now the guide uses cups of rice which is a very close approximation to weight. 1 cup is almost .5 lbs (anywhere from .44 to .46 lbs) from one source I found.
I would say the guide is close enough as the last addition in the guide expects the remaining rice to be 5lbs which would, if all things being even, be aligned to double the contents from 2nd step to 3rd step.
So in the guide the remaining rice is prob. more than 5lbs as expected.
If (and big if) my spread sheet has the understanding of doubling correct and my assumption the weights of rice (and koji) is of dry rice, then it should produce similar results.
Then again, I may be thinking about the ingredients being doubled not the volume. Have to checked to see if the volume from one step to the next is doubled?