Hello there,
It would be great to understand a bit better what sort of assumptions you are making on your cases, but it sounds like they are mainly related to the math and they drive you to mistaken answers.
For example, are you automatically assuming concepts such as “the market will continue to grow at the historical 5% yearly” or “demand will not decrease if the client raises prices”? If this is the case, you could do an exercise to proactively question yourself, for each part of the math problem you structure, was this statement explicitly stated by the interviewer / anywhere in the case? Does this statement hold true in every plausible scenario? If the answer is “no”, you may be making assumptions and you should let the interviewer know you plan to assume X or Y so that they can point you in the right direction. You can practice several math problem structuring mocks using this technique until you are able to spot assumptions naturally.
Hope this helps, and best of luck!
All the best,
Lucia