Rule 1 with any math is: Does this answer make sense? A lot of the things you do in aviation have mental math at the core. Avgas is 6 lbs/gal, distance is the product of rate and time, etc. So using these cues can tell you about what numbers to expect. If you get an answer that seems too big or too small, or just really off the answer you expected, go back and double-check that you did the problem correctly.