Estimation technical project is always one of the the hardest thing in Project Management .
There are so many moving factors that can easily affect the quality of Estimation, especially under certain conditions like time constrains and resource shortage.
Contingency
One of the good lessons I got is to add contingency into the estimation regardless. If we deliver things earlier than the deadline, that’s a good surprise. Otherwise, we still get enough buffer to get things done and deliver.
A good rule of thumb is about adding 50% contingency to your original estimation.
PERT Method
If you prefer more rational approach, you could try is with this PERT Estimation method. This is to give you a weighted estimation based on most likely, optimistic, and pessimistic estimations.
Extra Information
I knew you like to read books, this particular books is been recommended by couple of people to me around Estimation and Planning. Worth to have a read Agile Estimation and Planning if you get some time.