
James G. answered 08/28/19
Former PMP Certified IT Project Manager
For unforeseen events, you should be using risk based contingencies as specified in the PMI handbook So, 4 hours is not the right answer in most cases.. You should do one estimate at the end of requirements, which is plus or minus 25%. After design, you should know more, and be able to estimate it at plus or minus 10%. You should still have risk based contingency with an estimate of impact for each risk added into the estimate. This is based on what that risk would do to the project. The contingency also applies to schedule. You should consider having an estimating database for all of the work that you do. This could help you even if all projects are different. You can count complexity, modules, and more to have a formula to at least guide you based on past estimates and actuals. You should also use change control very strictly to avoid inflated scope without doing a re-estimate.. I hope that this helps. I managed projects from tiny to ten million and found success with these guidelines.