Calculating STC

I have been looking for a formula to calculate the STC for a series of different window/glass combinations. In my situation, I have casement windows with 1/4" overall IG. I can insert a second pane of glass at 2" using the location of my internal screen track or at 6" where it would sit on the sill of the window. I have taken outside measurements with a professional microphone to get the frequency and loudness time series for overflight departing aircraft noise. I found that the curve reproduces jet backblast, with the loudest noise between 30Hz and 250Hz. The loudest noise is 90db, but I often get 70db. Is there a formula to estimate STC for adding different types of glass (1 or 2 panes) with the different air gaps? What about getting credit for the glass being properly inserted into a frame with acoustic tape so there is no vibration?