Reading somebody else’s sheet

Start at the answer and work backwards. Find the cell that produces the headline number, look at what it references, and follow it back until you reach cells that are typed rather than calculated. Those typed cells are the assumptions, and the assumptions are where the argument actually lives.

Then apply two checks. Does the total behave sensibly if you change one assumption to something obviously extreme — and does anything fail to move that should have? A number that does not respond when it should is usually a formula pointing at the wrong cell, which is the most common error in any sheet that has been edited by more than one person.