I recently published a two part article on estimating time more accurately for web projects at thesambarnes.com

It describes the techniques I personally use when estimating which sound very similar to yours. But I think your article mentions something I didnt quite hit home enough in mine. That whatever you estimate, it WILL almost certainely be wrong! You should be aware of it, your boss should be aware of it and so should your client.

If you can foster that kind of realism amongst all team members and stakeholders you will find things much easier further along in the project.

All you can do is use your teams expertise, combined with your personal instincts, and deliver an estimate that is as accurate as possible.

Look forward to new posts :)

Sam