Where to measure?

I just had a question about where to measure my waist. I always thought that as a woman, my waist was the smallest area between my bust and hips (about 2 inches above navel). When my trainer does my measurements he measures right at my belly button. I try to use online calculators to get a rough estimate of what my body fat percentage might be and track progress. But there is a 3 inch difference between what I consider my natural waist and my navel. That's a huge discrepancy when I enter the measurements into the calculators. Just curious where I should be taking that measurement from. TIA!