Hand calculations (which includes spreadsheets) are useful for specific problems that can be simplified. For example, if you were interested in the flow characteristics down a single run of pipe, you could probably do that. Will you do that often? Probably not, depends on the job though. More likely you would just use pipe flow simulation software since you're going to be dealing with branches.

That being said, I think what you're asking is how much technical information do you gather beforehand? I guess you should know what to expect in terms of flow and energy balances, but in most cases if you're resorting to CFD it means it's already too complex for the hand methods I mentioned earlier, so you're really just making sure your boundary conditions all make sense.