I'd say around Dooku's level, or even stronger. After all he was confident enough that he thought he could take on Dooku's master alone. Granted that he had no idea Sidious was so powerful, he knew that he would have been stronger than his apprentice for sure, so he needed to be at least stronger than the count to even hope to have a chance against him. Maybe when Touching Darkness will be released next week we will have a better understanding of his abilities