Reading S/b values from Getsinger curves (semi-log graph)

Hi

I'm designing a combline filter and using Getsinger curves to find the ratio S/b of a line. The graph ΔC/ε against S/b is a semi-log one, the S/b axis being linear. I have calculated the ΔC/ε values and must read the S/b values from the graph. Must I convert the ΔC/ε value to log and then read the S/b value?

Thanks