When I was a candidate myself, I made a stupid math mistake (double counting) for my final round interview with a senior partner. Fortunately, it didn't lead to a rejection but I was asked to come back for an additional interview the following week where it was clear that the only expectation was for me to prove that I can do the maths correctly. I was a engineering student who had always been comfortable with numbers and so I knew a large part of it was bad luck and nerves as well.
It's hard to give generic advice but below are three things I did which I can reiterate having been on the other side of the table now.
1. Become comfortable 'running the numbers': I did the maths section of every case I could get my hands and practiced abstrat mental maths (e.g., long division). But most importantly, I started to force myself to see the bigger picture/dynamics behind the numbers than the explicit arithmetics (sometimes it's easy to focus purely on the digits when given large numbers and abstract percentages)
2. Develop your own communication style: depending on your comfort with numbers and under pressure, it's worth coming up with a style that works best for you. For some people, running the numbers independently then talking it through with the interviewer works much better than doing the maths as your interviewer watches. Additionally, often it's not neccessary to talk through and show every step of your thinking and there are logical shortcuts/rounding that can make a huge difference
3. Focus on nailing the 'so-what': the assessment is on your quantitative thinking NOT mental math. Drawing the implication or the 'so-what' from the maths is what differentiates you. Yes, that is contingent on you getting the maths right, but hopefully it takes off some pressure too...
Good luck and feel free to reach out if any questions!