Erosion grooves are known to develop on a surface of a planar magnetron sputtering target in the course of its use, causing the rate of deposition per unit power applied to the target (rate efficiency) to decrease with progressing erosion. It is shown that for a given target material there is a reproducible empirical correspondence between the rate efficiency and the groove depth. Once established, this correspondence offers a simple way for estimating thicknesses of the films deposited onto stationary and rotating substrates.