When adding postal code ranges start with the largest community, like the range for the state.  Set only the state tax. In most states this will include all taxes.  i.e., City is 1%... County is 1.5%... State is 7.5%... = 10%

However, if your state requires your taxes to be separated; you will only include the state portion.  i.e., State is 7.5%... = 7.5% 

Next add postal codes range for the county excluding the rest of the state.  Now, include all taxes for county and State.  i.e., County is 1.5%... State is 7.5%... = 9%

Next add postal codes range for the City excluding the rest of the state. This time include all tax levels.  i.e., City is 1%... County is 1.5%... State is 7.5%... = 10%

Important reminder: If the same postal code is in 2 ranges the range created last will override the earlier range.