I have two simple parameters I use to apply to all estimates. How many people are involved? And how many times has each person done exactly this thing before?

The more people involved, the more invisible work there is. Not just meetings, but handoffs and general coordination.

And things are only predictable if they are done to the point of being known things. Rehearsed activities are known activities. Even if you shake up a part of it, people that have rehearsed are more capable of dealing with the shake ups and keeping things moving.