I'd start with your clean max as your RDL max for now, assuming it's a full squat clean. In a week or two, you'll have a better idea what your max numbers are.
But 120-150% of your clean is also a decent starting point if you find your rdls are too easy. Again, you can always adjust it upwards if it's too easy. But since you haven't ever really done them, ease your way in over the course of a couple of weeks.