exchange rates for project appraisal

hi sir usually when we have to find the NPV of any particular project in the home currency we calculate the exchange rates using PPP. those questions usually mention in the scenario something like "these rates(inflation rates) are expected to be constant over the term of the project" (with ref to Q:Blipton int) my question was, if a question mentions that the inflation rates or interest rates( in the case of IRP) WONT be constant over the term of the project, in this case do we calculate the spot rates still normally using the formula or is there any other way? so far i have not come across any question that said that. so maybe im over thinking this point, but i just wanted to confirm with you incase i ever face something like this in the question.