I recently made a RIS and added one pint (375mL) of Makers Mark to the secondary. The bourbon flavor is not nearly as strong as I thought it would be. It is there, but it does not take over or dominate. A Dubbel is not quite as roasty/rich as a RIS, so you may want to tone that amount down slightly to compensate for the lighter flavors of the base beer.
"Give a man a beer, and he will waste an hour. Teach a man to brew, and he will waste a lifetime!" Bill Owen