A crystallography row starts from a checked plane-family 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
A Miller family fixes a plane spacing
The training lattice constant is 15 m. The family is 2, 2, 1.
h2+k2+l2=9
The exact spacing is five metres
The denominator is three, so 15 m gives spacing 5 m.