You need to look at each home and area individually though. Sometimes the appraised value is quite a bit more.
The tax assessment is determined by the muncipality so they have enough money to pay for schools, and streets, and all the stuff we get with our property taxes. This number is strictly a buget number to ensure enough tax base for an muncipality.
There is no formula that the says sale prices is "x%" more or less than assessment.
The only way to determine value/sale price is comparible sales in the area, and negotiating a deal with the owner.