It is a reality that no matter how similar two projects might appear on paper, because of so many different factors affecting each job, your costs and scheduling will differ. Therefore when keeping your historical costs and using it, rather than relaying on totals, try to have as much detailed data as possible in terms of unit costs, for instance time and labor required to finish a certain part of work. and make up your next estimate based on these cost data. This will enable you to make much more accurate estimates. It all comes down to the detail you need when doing a particular estimate. For many budgeting purposes though, you will not need this much detail.