The rule of thumb I’ve seen is to regard a school as a safety if it has 80%+ admit rate, and the student’s stats are above the 75th percentile in all areas that can be compared (such as unweighted and weighted GPA, both parts of the SAT, etc).
Of course this doesn’t hold in the case of impacted majors that have a much lower admit rate than the school as a whole (such as CS, etc), or majors that require an arts portfolio or other special process.
And, a school isn’t really a safety unless it is also a financial safety.