Exactly. And economists live by assumptions all the time. You know the old joke about “assume a ladder”, and that's what economists do.
But seriously, if you look at appropriations estimates, divide them into two categories. One I'll call fixed appropriations—you're appropriating a sum to an administrative unit for its running costs. Assumptions are not important there. The other thing, our open-ended programs, mostly in standing or permanent legislation, constitute by far the largest share of the government's budget in Ottawa. Those are driven almost entirely by assumptions. What will the price level be? What will the employment rate be? What will the participation in a program be? Government announces a new program with respect to assisting low-income people with nutritional needs. You put an entry in the budget. But what matters is the assumptions you make as to who will participate, to what extent, etc.
So in the fixed budget, assumptions are not important. In the much larger, open-ended budget, and of course for revenues as well, assumptions are critical.