I would set up just a few variables:
Dependent variable, Y: growth in state GDP
X1: National GDP (controls for all exogenous variables)
X2: state tax rate
enter all your data for all 50 states for all available years (could be 500 observations for 10 years!). Y (state growth) = a1 + b1*National growth + b2*state tax rate. Coefficient b2 becomes the effect of the tax rate!