Last week was very important in my life span as a Software Tester. I realised that no matter how precise you are at estimating efforts and schedule - estimates are just that - Estimates - rough idea initially, but as you refine it - you get a more solid view of what is realistic estimate.

I Was surfing on QAForums -> Estimation & Planning section and read that estimation is all about experience. Tools and Templates help you to some extent - calibration needs to be done by you.