You're right. My remainder calculation in my code snippet is incorrect. It should've been a floating point remainder instead.

    import math
    
    def tax_f5(amt, rate):
        t = amt * rate * 1000
        return round(t) // 10 + ((math.fmod(t, 10.0) - 5.0) > -1e-7)

But then since there's now an epsilon, it raises the question of how many digits of precision the tax rates typically need. This is indeed a difficult problem.