That calculator is tremendous. I really like how it shows you the locations in the room where those modes will be the loudest. I will revisit my sweeps and see if I might be able to better configure my room treatments. I suspect you could tweak the room size values to match the peaks on your REW data, and then the predictions of mode locations would be spot on. I know where my sub has the most output in my room, so if the model agrees then I see no reason I wouldn't be able to trust the rest of the predictions.
Another good tool is a spectrogram. It gives you the distance of the reflections and their frequency range so you can narrow down which reflections are the problems and then choose the appropriate type of treatment.
In this image, you can see 2 strong vertical strip reflections at @ 4ms and 9ms respectively that range between 1KHz and 7kHz respectively.
Those distances correspond to my side and cross-side reflections even though I had OC-703 broadband absorbers in those locations documenting they weren't doing a great job. I replaced the OC-703 based traps with horizontally mounted 180 degree dispersion designed polycylindricals of the same size which evenly distributed those strong first reflections vertically which erased/attenuated both reflections without taking the energy out of the room. There are cord calculators available to design various poly dispersion patterns.
In this case, a solid poly worked better than porous absorption and significantly stabilized the soundstage when twisting your head. The leading ear no longer switches between the side and cross-side reflections.
