How important are the maths at the case interview really? Let's suppose that when you reach the maths part you (i) explain the steps you are going to take (ii) ask for the data that is missing and you need and (iii) once you reach the arithmetic part you mess some numbers due to pressure (i.e 2+2 = 5)
Is such a big deal? I think it's more about how you structure the problem and whether you identify the data needed than actually performing perfect calculations....