A toy Bragg check is a path-difference equality. 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 first-order toy Bragg fit compares one wavelength with two spacings
The plane spacing is 3 meters and the wavelength is 6 meters.
d=3m,λ=6m
The equality closes exactly
Order 1 gives the left side 6 meters and the right side 6 meters.