It's a balance. And unfortunately, there is no easy way to know when to stop decomposing work when estimating. You have to do some otherwise you are simply guessing on the top-level features. But we can't even guess how long it will take to do something without a detailed design, which in itself would take a lot of work to estimate. Even civil engineering, which deals with far more concrete (pardon the pun) entities than software, has seen projects go ten times over estimate.