If you build a general ledger application for the 10th time, sure forecasting is fairly straightforward. Nothing I do at my (very large, non tech but highly software driven) employer has ever been done before here. All estimates are treated officially as if they are date time accurate, but changes happen during the lifetime of the project so often you may as well use a random number. I call it a "nod nod wink wink" estimate: every wants it to be accurate, but no one really expects it to mean anything, other than the budget people.