Estimating jobs...

I work with estimators in commercial construction every day. Some of them can get a job really close (as little waste as possible) others end up with multiple lifts of board laying around the jobsite and bundles of insulation all over. One thing they all do is over estimate the amount of time it will take their guys. That way if things go wrong they are still safe but if things go well then they can wrap up the job early and it makes them look good. If you get a job that will take 20 hours then estimate if taking 25. If you get it done in 20 then bill them for 20 and not 25. You will have a happy customer!