The transit depth is the fraction of starlight blocked, depth = (Rp / Rstar)^2, the square of the radius ratio. Squaring comes from comparing areas of two disks seen face-on. The exact value assumes a uniformly bright stellar disk and a dark, fully transiting planet; real light curves add limb-darkening, noise, and radius uncertainty.
A transit depth starts as the square of one radius ratio. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.