Software Estimation: Demystifying the Black Art (Best Practices (Microsoft))

Don't provide "percentage confident" estimates (especially "90% confident") unless you have a quantitatively derived basis for doing so.

Avoid using artificially narrow ranges. Be sure the ranges you use in your estimates don't misrepresent your confidence in your estimates.

If you are feeling pressure to make your ranges narrower, verify that the pressure actually is coming from an external source and not from yourself.