+1. That's as good an endorsement as one could hope for.
Unfortunately, most if not all of the free apps suffer from a lack of vetting by independent engineers, and in some cases there are persistent questions about their accuracy. None of them that I'm aware of is open source, so it's anybody's guess what they're doing. My program is open source, but it hasn't received enough attention to know whether my approach is valid or not, apart from my own confidence in it. And it's far from what I'd call "software."
If you want formulas, I've got formulas.

My "non commercial" web page includes some documents where I derive the equations for the basic response curves from laws of physics. Long ago, I created formulas that could be entered into Excel, but they're quite hairy because Excel doesn't handle complex numbers. This is one reason why I switched to a general purpose programming language for this kind of calculation.
For me, this is more of an academic exercise than an attempt to re-invent speaker design. Turning my equations into a primitive app, and using it to build real speakers, are "the proof of the pudding" for whether I understand this stuff or not.