That they do.
It's always hard to say, from a distance, especially with kettle cooks. It really depends on cooktemps. What your cooktemp therm says depends on the therm itself, its placement, and so forth - so it can be hard to determine upfront. Once you do one or two and see how it goes, you should be able to replicate the cook without too much difficulty, increasing or decreasing the cooktemps if desired, and thus hone in better on projected cooking time. At first, imo, it is better to allow plenty of time. As noted, you can hold a too-early-finished butt without problem - and that's better than keeping everyone waiting, no?