How to obtain a percentile ranking for a journal? (i.e., top 30%; 30% to 50%)
Likely this is not the answer you want to hear, but I assume the administrative service you talked to is correct. If nobody really knows which journals fall under which of these (rather subjective) categories, it will not be feasible to in advance establish for sure way how much any paper you write will count in the end, when you are evaluated.

This is not nearly as strange as it sounds. Formal rules for promotion and the evaluation of research tend to live in the ugly grey area between bureaucrats who like things orderly, well-defined and, most importantly, written down, and the reality of science, where basically no two scientists will ever be able to agree on an absolutely consistent quality ranking of publication outlets. What usually happens is that some sort of nebulous rankings are defined without specifically ranking concrete venues,and then leave it to a commission of academics to decide ad hoc which publications fall into which category.

At my current university, we have similar nebulous requirements for PhD graduation. Essentially, it is required that students need at least one A-ranked publication. However, what is considered A-ranked varies considerably between different faculty, and is basically negotiated when graduation time is near. I assume requirements for tenure etc. are handled similarly.

Note that there are some initiatives that are trying to rank publication venues more formally, for instance CORE for computer science. However, these rankings are also far from perfect (or, for some fields, they are apparently downright terrible).