Ah, I see you've worked on financial software as well ;)
We've definitely had our fair share of "IDK what to tell you, those guys are mathing wrong".
TBF, though, most customers are pretty tolerant of explainable differences in computed values. There's a bunch of "meh, close enough" in finance. We usually only run into the problem when someone (IMO) is looking for a reason not to buy our software. "It's not a perfect match, no way we can use this" sort of thing.
The super-advanced math that finance folks would throw at us was daunting.
At this particular job I used the plus, the star (multiplication) and once I even got to use the minus.
There's a legend going around that a friend of mine has used division, but he has a PhD.
I had to explain recently that `a * x !== b * x when a !== b`*... it is infuriating hearing "but the result is the same in this other competitor" coupled with the "maybe the problem here is you're not knowledgeable enough to understand".