@sjvn This is kind of an optimistic approach to how real projects are handled in the first place!
Most of those I have seen (or where I've dealt with the aftermaths of them) have been like: Whatever they say it will take, change that up to the next unit magnitude, then double the physical quantity and expect them to charge 10 times what they originally said.
So, they won't try to fry a chicken at 300° F for 3h. It will take them 6 days. (The applied heat will grow exponentially during that time: First they will start with close to no heat at all, and then crank it up at 5 days 23h to bloody-f-in Nuke! +1)