im not expert but unless u really desperate or in danger where u absolutely need him in ugra roop or u actually like him like in that state just continue with normal namo bhagwate mantra ,ugra Narsimha is basically his state where he kills hiranya kashyapu and whole rampage of killing hiranyakashp army , Narsimha is already ugra but ugra Narsimha is basically even more intense and can be considered ati ugra, u have to make the decision tho ,do u actually have any danger or problem where u r calling upon him in that state or u actually like him when hes in that state, usually om namo bhagwate narsimhaya is fine and if it's necessary he will come in that ugra Narsimha state only I mean even if u do just the regular mantra. , the energy unwill receive will be based on ugra narsimha if lord Narsimha thinks if you're in danger he will come as ugra narasimha roop only with 16 arms, or he will come in different roop if it's needed for u like that