Holding signal fixed isolates the half-signal constant in the occupancy ratio. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Half-signal four gives three fourths bound
Hold signal at 12. Half-signal 4 gives occupancy 3/4.
12/(12+4)=43
Half-signal six gives two thirds bound
Half-signal 6 gives occupancy 2/3 with the same signal.
12/(12+6)=32
Half-signal twelve gives one half bound
When half-signal equals the signal, the toy occupancy is 1/2. This is a ratio check, not receptor chemistry.