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.
"After the first glass, you see things as you wish they were. After the second, you see things as they are not. Finally, you see things as they really are, which is the most horrible thing in the world."
--Oscar Wilde on Absinthe