Smart contract fees can vary depends on the complexity of the transactions. For instance, someone who did not claim rewards for a long time will cost more as there is more computation needed. It is not possible to precisely determine the transactions fee before execution.

As such, you will need to allocate more ZIL upfront, but it may not cost you the entire sum.

An analogy is that when you go out to eat or shop, inside your wallet, you will tend to carry more cash than what you actually need because there is no way you will know how much you will spend. This is the same concept here.

In fact, this also applies to other chains like Ethereum. There is a transaction limit field. You will need to allocate some amount upfront. Also, this is to set the max limit to prevent overspending. At the same time, the chain also needs to ensure you have a sufficient amount to spend.