Usually estimating large jobs by the foot ends up being close to the same as hourly. Can figure it out both way and go with the higher estimate as to cover any thing unforeseen, and not kick yourself later when you feel you undercharged. It's easier with most clients to estimate high upfront, rather than ask for more after the job is done. That said being forthright with people about a job possibly costing more usually makes them more excepting a higher price at the end. Making sure you know exactly what the client's expectations are make it easier to estimate any job and allows you to exceed the expectations without losing money.