A small Miller family gives an exact plane spacing. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Miller indices choose a plane family
The toy cubic cell has side 6 metres. The family uses indices 2, 1, 2.
22+12+22=9
A perfect square keeps spacing exact
The square sum is 9, so the denominator is 3 and spacing is 2 metres.