In his terrific book, Home Studio: Build It Like the Pros, Rod Gervais included links to a collection of very useful spreadsheet tools. I collected them into a single spreadsheet, labeled the tabs, and gave that to my MSCM students: New Gervais Tools. Rod provided us with calculators for Sabin RT60, panel absorber design, mass-law transmission loss, room modes, and quadratic diffusors.
There is a whole folder of acoustics tools, including this tool and many others, called "Acoustics." If any of that is useful to you, go for it.