The said method of intention is not right. If your heart confirms that you had missed some fasts though their exact number is not known, then you only need to make the intention of missed fasts, and do not suffix it with the intention of nafl (otherwise?..). And when you had kept missed fasts to an extant that your heart feels satisfied that you have finished all missed fasts, then you can keep with the intention of nafl. And if you wish to obtain the virtues of Arafah, Aashurah etc, keep particular nafl fasts in these particular days, and complete missed fasts later as much as your heart deposes.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband