Hi Jyotishman,

as for structuring, the best preparation I found for math-intensive case has been on consulting handbooks of top MBA school. In particular, I would suggest the following:

1. Download a couple of handbooks for top universities; there are several free available online (INSEAD, LBS, Columbia, etc)
2. Skim cases trying to identify those with more quant based approach (usually you can identify them visually, as they involve graphs)
3. Read the questions of the cases; try to answer first with your approach, then read the solution
4. Write down all the mistakes you do when solving them
5. Starting at least one week before the interview, review daily all the mistakes redoing the questions you found more challenging

As for math speed and accuracy, you can find several resources online where you can test yourself under pressure. PrepLounge and Victor Cheng websites in particular have an ad-hoc section for that; you can check them at the following links:

https://www.caseinterview.com/math/home.php

https://www.preplounge.com/en/mental-math.php

Hope this helps,

Francesco