How to determine enzyme inhibitor efficacy?
If the inhibitor is a competitive inhibitor, to quantify inhibition the $K_m$  of the inhibitor ($K_I$) is determined.

$K_I$ is compared to the $K_m$ of the substrate.

Then, the degree of inhibition is a function the inhibitor and substrate concentrations.

$V_{max}$ is unaffected by a competitive inhibitor.  

In the presence of an inhibitor the kinetics are modified such that the enzyme has an effect $K_m$ as:

$K_m(1 + [I]/K_I)$