I'd take the real world Louis XIII of France as an example. Not only was his father simply known as Henry IV but both he and his son were also monarchs of the Kingdom of Navarre. ((it joined with France by personal union in 1589 when King Henry III of Navarre inherited the French throne as Henry IV of France, and in 1620 it was merged into the Kingdom of France.)) 

So Louis XIII King of France was also Louis II King of Navarre, son to Henry IV King of France, who was Henry III King of Navarre. 

In short? You can probably add whatever numbers where you want and be completely cool, so long as you make sure that our King Erasmus VI didn't come after an Erasmus VII or some such. Make sense?