Have you ever tried to guess how many pieces of candy there are in a jar? Or tackled a mindbender like: “How many piano tuners are there in Chicago?” Physicist Enrico Fermi was very good at problems like these -- learn how he used the power of 10 to make amazingly fast estimations of big numbers.
The accuracy of order of magnitude estimations comes from the balance of under estimates and over estimates. Work through a few Fermi questions and purposely skew some of your responses. Does this affect your final estimation?
The University of Maryland maintains a site dedicated to Fermi Problems. You can see them here: http://www.physics.umd.edu/perg/fermi/fermi.htm
Another example of a Fermi-like-problem is the Drake Equation. It tries to estimate the number of intelligent life in the universe. See a TED-Ed lesson about it here: http://ed.ted.com/lessons/calculating-the-odds-of-intelligent-alien-life