Cascaded squared ratios multiply stage by stage. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The first cutoff stage halves the squared budget
Input budget 20 becomes 10 after one cutoff stage.
20⋅21=10
The second stage multiplies again
The second half response makes final budget 5, not ten plus another correction.