First, the primary and secondary blockholder refers to the bidding process - both are blockholders, but one gets the block in the primary phase of the bidding and the other gets the block in the secondary phase. Generally, though, the more senior will get a block in the primary phase while the secondary blocks will go more junior.
So if you know the hire date of the junior secondary blockholder in a base (I think that's the hire date they give in the newsletter), everyone junior to that (hired later) is a reserve (although LODO's may be different - didn't have pilot LODO's....). So just making up dates, if the junior secondary blockholder in CLT was hired on 8/1/1990 anyone with less than 17 years would be reserve. They may not have been reserve the whole time due to the cycles of growth followed by downsizing, but they would be a reserve currently.
With the multiple bases the DOH of the junior blockholder will be different from base to base. PIT probably has the reserve with the most longevity because of all the cuts there, while LGA/BOS reserves probably have the least longevity - but that's just guessing on my part.
Jim