With experience comes knowledge. For remodel jobs (which I do lots of) I ususally ballpark it a little higher than what I think it will cost. I doo the work t&m billed at $78.00 plus materials marked up 30% (percentage subject to change according to customers attitude, job conditions etc). Most times the actual charge will come in right at or slightly below the estimate. Customers love to hear the job will be less than expected.