A perfect mirror uses the same source but a doubled pressure multiplier. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
A perfect mirror doubles the pressure
The mirror case uses the same irradiance and speed with multiplier two.
pmirror=2cI=2Pa
The absorber value stays one pascal
The doubled value is not copied from the absorber label.