The book "Summit Routes: Washington's 100 Highest Peaks" (authored by Scott Stephenson and Brian Bongiovanni) lists Apex Mountain as the 100th-highest peak in Washington.
The book uses the following criteria, for making that list:
-> Summits must have at least 400' of clean prominence.
-> No volcanic sub-peaks are used except Little Tahoma, due to its individual size and prominence.
-> Seven-Fingered Jack and Blackcap Mountain were included because their clean prominence was within a small error margin that might make the 400' prominence criteria.
Using these criteria, the book authors determined Apex Mountain is the 100th-highest peak in Washington. It is amazing how many different criteria there are to determine the highest peaks, mostly because of volcanic sub-peaks but also because of error margins.