Holding shear rate fixed makes the viscosity contrast honest. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Hold the shear rate fixed
Both cases use speed difference 8 m/s over gap 4 m, so the rate stays 2 reciprocal seconds.
8/4=2
Double viscosity, double stress
Changing viscosity from 4 to 8 Pa*s changes stress from 8 to 16 Pa.