I would set up just a few variables:

Dependent variable, Y: growth in state GDP

independent variables:

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!