Assuming something is absolutely fine. In fact astute and calculated assumptions help us in moving forward with greater agility. Assumptions are made based on prior experience and from information currently available at hand. Gut feel, rough estimate and assumptions strongly twine with each other. When people say ‘don’t assume anything’ they are downplaying the power of assumption.
Assumption needs to be verified against data asap so that sanctity of assumption can be checked. If assumption is found to be wrong then the premise which helped in reaching that assumption should also be checked so that we won’t use that premise in assuming something else. If assumptions are not cross checked then human mind will take those assumption as base facts after certain time interval and assume something on the top of the existing assumptions. Then deviation multiplies and probability of entering into dangerous assumption zone increases.
Go ahead and assume. Then verify.