One equation that can be used to plot a curve of % inhibition versus inhibitor concentration is the Hill equation. Assuming zero % inhibition in the absence of inhibitor,

% inhibition = MAX[Inh]n/(IC50n+[Inh]n) (Note: % inhibition is 100-% activity)

MAX is the maximal % inhibition, which is usually 100%, although it doesn't have to be.

IC50 is the concentration of inhibitor, [Inh], at which there is 50% inhibition.

n is the Hill coefficient of cooperativity. In the simplest case, it has the value 1.

It is not a good idea to try to extrapolate, especially since you have only 3% inhibition at 0.1 µM. What you need to do is measure the % inhibition at a range of concentrations of inhibitor, then use a nonlinear regression computer program to fit the % inhibition versus inhibitor concentration data set to the above equation (or an equivalent equation). From this you will obtain best-fit values for the parameters MAX, IC50 and n, and you will have a clear idea of the relationship between inhibitor concentration and extent of inhibition.