You're thinking of this the wrong way (and, not to put too fine a point on it, NOT in the way project management professionals do). A critical path project has

You're thinking of this the wrong way (and, not to put too fine a point on it, NOT in the way project management professionals do). A critical path project has a chain of tasks. Task A must complete before B which must complete before C, and so forth. If A is delayed, but you've turned off the critical path engine by making C a "manual task", then C's forecast dates *WILL NOT REFLECT* the anticipated move to the right as a result of A's delay. This is a HUGE problem. You rely on forecast dates being live, and showing true projections of reality (or as true as possible). You can even seat deadlines, and generate alerts if a task isn't going to meet one. But if you nail something down because it quote-unquote can't slip, you're preventing the software from telling you when a slip is coming. That is very, very, very dumb. It's also entirely predictable that someone on HN would try to tell me it isn't. And yes, you can put literal numbers in spreadsheets. But bad things DO happen when people represent values as sums (or as otherwise calculated based on other values) when they are in fact not. That's what "manual tasks" are.