I am trying to find out whether the number of participants I recruited (N=42) so far is sufficient for my study in which: there are two interfaces (say A and B). ALL students in the study will try the interface, A, in the first week and then the interface, B, in the second week. The results from both attempts will be rated by experts in 1-5 scale (where 1 being very bad, 3 being neutral and 5 being very good). Alpha is 0.05. And effect size 0.5 (Cohen). The null hypothesis is that there is no difference between the outcomes (as rated by experts) between interface A vs. B. I visited online power/sample size calculators like this and this, but I don't know the "mu" of the population nor am I sure that my design is one-tailed or two-tailed (although I'd guess it's one-tailed? Correct me if I'm wrong).
I'm not very familiar with statistics, so I need help from more knowledgeable people like the ones from this community. Thank you for your help in advance.