Net Assets in ROCE/RI

Respected Sir, hope you're having a good day.. I'm very confused as to which figure to use as Capital employed, should it be Opening net assets, Closing Net assets? or Average (in case the info is given) I've come across different pastpapers doing it differently with no mention as to which figure to use in the question. Also, in the MCQs where Average could be calculated they've used opening net assets instead. Please tell me which one to use?