Estimating is not "guesstimating." Since the units in an estimate are dollars, every estimate should be as accurate as you can make it. If it isn't, you are gambling just as surely as if you put your money on a horse. All estimates should be double-checked, especially the extensions. An additional sight-check should be made to be sure there is no mistake in extension. Because it is your livelihood, an estimate cannot be checked too often. You rise and fall by the price you put in. And once that price is down in writing and a bid bond or security of guaranteed performance is attached, it is too late to change your mind. Checking must be done before the number is written. There are rules of thumb that can be used in checking the overall estimate. Certain types of concrete, experience will tell you, should cost so much labor per cubic yard. If this figure varies greatly from what your judgement tells you it should, then a review of the plans should be taken to find out why. If, in your opinion, the figure happens to be extremely low, you may find there is a large amount of concrete on the ground, concrete that does not have to be formed. In other words, since the ratio of forms to cubic yards of concrete is small, the concrete should be cheaper. The local labor situation can also mislead an estimator as to costs. It should be surveyed carefully in advance of making the estimate. Particular attention should be given to unions and their working rules and how these affect the necessary behavior of a contractor in a particular community. Other local conditions must be taken into account. The estimator needs to know what sales taxes are applicable and what type of work is exempt or not exempt. Permits, for example, can run tremendously high in many towns.