A deterministic chemotaxis toy model treats contrast as a run-count bias. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Use integer run counts
The cell has 5 forward runs and 3 backward runs.
5+3=8
Excess over total gives the bias fraction
The excess count is 2 out of 8, so the bias is 1/4.