[Silhouette]

Long story short you always end up billing by time, except for some short specialized work you're very confident you can handle.

Or the other end of the spectrum, where you're talking about year-plus projects with year-plus-sized price tags, and it may be worth the overheads for both sides to agree a starting spec, timescales and milestones up-front. You do need to make sure you also have a clear change management process in place, including a shared understanding that any changes are subject to approval by the developer and will increase the bill and timescales.