Level of Complexity (Score 1)

Divide and rule policy not only helps to understand the system better but helps in the estimate as well. Added to that if you assign the level of complexity for each tasks it will be help in providing better estimates. Assign the complexity levels to the atomic level of design as more difficult,hard,normal,easy and then assign appropriate times\days to each one of those. You can further decompose difficult tasks into easy ones to derive estimates.