In complex billing systems (that are also often responsible for computing non-trivial taxes), using fixed point math with configurable rounding rules -- is a mu

In complex billing systems (that are also often responsible for computing non-trivial taxes), using fixed point math with configurable rounding rules -- is a must. There are requirement time discussions/rules that indicate if a computation has a rounding rule or not (and it is normally driven by whether a calculation involves division, or if a computation must match an amount in a different currency). Dividing amounts, as example, always comes about when there are mid-cycle cancelation/onboardings to a service. Currency translations cause that as well.