And why they don't work
Estimate the worst case scenario, and multiply it by 3.
Slack: Time to refactor or to compensate
A feature takes N points to implement.
How long?
N / 2 (max)
How long?
N / 2 (max)
How long?
N (max)
How long?
N (max)
[considering real time, not effort]
How long?
N / 2 (max)
(not considering respecs, approach changes, etc)