Your loan-to-value, for the purposes of this calculation, is your current loan balance divided by the LOWER of the original purchase price or appraised value. You would have received an appraisal when you got your FHA loan.
Assuming that it came in at $200,000, and that you paid $200,000 for the property, you'd be able to cancel your FHA policy when your balance hits $156,000.
Currently, your home equity is 89 percent ($200k / $178k). However, if your home has increased in value, you may be able to drop your mortgage insurance by refinancing to a conventional (non-government) mortgage.