Imagine doing the task with the wrong / outdated tools and base your estimate on that. e.g How long would it take to build the fort using only a hand saw and screw driver. (50 cuts at 5 minutes each, 200 screws at 1 minute each etc). Now build it with power tools and you have plenty of time to deal with the stuff you didn't plan for etc.

For an ETL project you could think in terms of Sql 2000 dts and then build it with SSIS