However, if a state uses federal taxable income as its starting point, then those federal changes will automatically flow through to the state's taxable income, as well. (fool.com)
The amount excluded under either method can not exceed 40 % of federal taxable income. (tax.vermont.gov)
Initially, all state and local taxes not directly tied to a benefit were deductible against federal taxable income. (taxpolicycenter.org)