If you know that wavelength is inside the frequency sampling points, then you can first search that frequency:
find the index from "f", and using the same index to get T.
rg, fn=find(f,f0) where f0 is what you want f0=c/wavelength
then ?T(fn) is its transmission. If f0 is not at the sampling point, it interpolate it.
If you want extact value, you can specify the wavelength in the monitor using "custom" frequency points. Or you can add another monitor with that specific wavelength.
But this question is not within this topic. Please write a new post whenever you have questions.