However, if we assume a 3.5 seeker/opening ratio then 600 applications/opening implies 171 applications per seeker, which sounds too high.
Looking at the methodology, it's clear what the problem is: the denominator in the official seeker/opening ratio is simply the number of currently unemployed people. Those 600 applications must include people who are working, perhaps part-time, so technically not unemployed. I think your estimate sounds about right.
If there are 600 applications per listing, and each applicant sends 50 applications, then there are 12 times as many applicants as listings.
Do the 600 and 50 numbers seem about right?