I am a language editor of academic papers in civil engineering (typically written in English by Poles).
I frequently encounter the term 'assumption' being used to describe a value that is adopted for the sake of calculations. For example, an author might not precisely know the original thickness of a corroded steel shell of a silo, but makes a decision about what it might have been in order to perform calculations/modelling. This would typically be written as something like: "assumption for thickness of steel shell = 6 mm".
I really feel uncomfortable with words like 'assumed' and 'assumption' on the basis that they are, almost by definition, unscientific. What would be the correct way to describe the above example?