Estimating time is indeed difficult. We've had success with applying a
strict method of using fixed amounts of time and breaking tasks down. We
estimate in man hours, and use a fixed scale of units: 1, 2, 5, 10, 20,
50, 100. The rule of thumb is to pick the higher of the two if your
estimate lies between two. If unsure, divide and conquer - break down
the task and estimate each subtask.
It's better with a mix of 10's, 20's and 50's than multiple 100 hour
blocks as the risk of error stands in proportion to the size of the