To measure thin film thickness by UV VIS transmittance/reflectance you should know the film refractive index (in general both real and imaginary part, if not absorbing you may limit to the real part). In general what you measure by recognizing interferometric fringes is the "optical thickness" of the film. Also pay attention to the effect of your substrate. I would recommend you have an optical model of your stack (both substrate and film) and fit it to match UV-VIS values.
In my opinion best way is by a stylus profilometer.
I would use AFM if you would be able to etch the film and get a sharp edge to get second confirmation in addition to profilometer.
I don't think that the results obtained by the UV-Visible are sufficient to measure the thickness, I believe that the use of polarimetry or the elipsometry methods are necessary,
But you can use the enveloppe methode from the reflectance and transmittance curves to calculate the index and thickness, this method is not accurate since the reflectance value given by the UV-VISIBLE was obtained indirectly.
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