Power analysis for Cox PH model


I am new here but have searched the forum before posting.

I am planning a retrospective review of previously collected data. N=480.

Going to use a Cox PH model and need to conduct a power analysis.

I have NOT run any analysis yet so I do not know the survival times in each group, nor do I know the effect size. There are two groups in my analysis, Group 1 n=72, Group 2 n=408.

ANY help would be very much appreciated. I can use G*power or R with the powerSurvEpi package.

Thank you!!!
Any help would be truly appreciated. Stated differently, how may I perform a power calculation a priori for logistic regression? I only have data available from other studies which indicate that the independent variable I am putting in the model has no effect.