When you provide an instrument, it is the Ramsey planner who chooses this value to maximize the objective. That is why you have one equation less than variables. Your job is to provide a steady state conditional on the value of the instrument. Think about it this way: Dynare gives you a value for T. Taking this value as given, you have to tell Dynare the analytical steady state for all other variables. See also [Ramsey Policy)
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