How do you manage project estimation and managing your teams time

Question

I'm thinking of things like:

• designating a developer having say 6 hours out of their normal 7.5 hour day dedicated to the project, the rest for other work/company related activity (meetings, emails, calls etc), maybe a senior dev is less.
• building in 20% contingency time to estimates.

What are the best ways to produce tight estimates and at the same time shield developers from management.

Addendum: I've been asked to help my current client with these types of issues. They are a really poor dev team with the business calling all the shots, scope creep, overlapping environments, priority changing at a whim etc. I want to help them get better.