I have no idea where people draw the line, but many of the recent questions about "how do I calculate this investment number" strike me as being a better fit there than here, since they're based on the assumption that the formula has some predictive value and to me that means quant.
Basic stuff like apr and history is within our remit. I'm not convinced anything much deeper is.
But I'm not convinced it isn't either.
It's all fractals and fuzz at the dividing line, but we should be able to at least state a principle, and maybe publish it so folks know which stack to start in before being redirected.