thickness of diffusion layer

in electrochemistry
https://doi.org/10.1351/goldbook.T06342
The distance from the electrode where the ratio $\frac{c\,-\,c_{\text{e}}}{c_{0}\,-\,c_{\text{e}}}$ reaches a given value. ($$c_{\text{e}}$$ is the @I03083@, $$c$$ is the concentration, $$c_{0}$$ is the value in the bulk solution.) If this ratio is selected as, say, 0.99 the corresponding thickness of the @D01716@ layer is denoted by $$\delta_{0.99}$$.