If your guess is a mathematical formula with assumptions for the inputs, those assumptions can be tested and the formula refined ... It's a SWAG.
You use things like Average/95/99, t shirt sizing, how much work can be done in parallel, 3rd party/client dependencies, ramp up time for a new library ... it's still a wild assed guess but at least there's some basis in reality and you can defend your estimate - show me what I'm wrong about or missing and I'll revise my formula.
Negotiating over the assumptions and inputs is 90% of the battle anyway. Without a model you're standing on sand.
You use things like Average/95/99, t shirt sizing, how much work can be done in parallel, 3rd party/client dependencies, ramp up time for a new library ... it's still a wild assed guess but at least there's some basis in reality and you can defend your estimate - show me what I'm wrong about or missing and I'll revise my formula.
Negotiating over the assumptions and inputs is 90% of the battle anyway. Without a model you're standing on sand.