"You might think you already know what an estimate is. My goal by the end of this chapter is to convince you that an estimate is different from what most people think. A good estimate is even more different.
Here is a dictionary definition of estimate: 1. A tentative evaluation or rough calculation. 2. A preliminary calculation of the cost of a project. 3. A judgment based upon one's impressions; opinion. (Source: The American Heritage Dictionary, Second College Edition, 1985.)
Does this sound like what you are asked for when you're asked for an estimate? Are you asked for a tentative or preliminary calculation—that is, do you expect that you can change your mind later?
Probably not. When executives ask for an "estimate," they're often asking for a commitment or for a plan to meet a target. The distinctions between estimates, targets, and commitments are critical to understanding what an estimate is, what an estimate is not, and how to make your estimates better."

Primeiros parágrafos do livro "Software Estimation: Demystifying the Black Art", da Microsoft Press, que você compra aqui na Amazon por meros 50 reais.

É tão importante deixar claro que uma estimativa não é um contrato, não é um compromisso, que o cara abre o livro dizendo isso.

Quantas pessoas e empresas entendem isso?

Acho que esse parágrafo, por si só, resolveria 90% dos problemas comerciais decorrentes de mudança de escopo e desvio de prazo presentes no mercado hoje.

Estimativa == Cálculo aproximado.

Combinado?