The best estimation technique I've seen is ROPE: Realistic, Optimistic, Pessimistic, Equilibristic. It's fast to ballpark, effective solo or with teams, great for PMs and managers, and able to go directly into critical chain scheduling or monte carlo simulations.
Ha! The name ROPE is because a rope is a group of strands, braided together into a larger and stronger form with higher tensile strength.
With ROPE, the four numbers combine together to create a larger and stronger estimate. I do estimates for clients, and ROPE provides a way for each stakeholder to see that estimates are really ranges of probabilities.
https://github.com/SixArm/sixarm_project_management_rope_est...