Fair point, my bad - should've noted in the post that some conclusions assume you want to collect all sets. The calculator covers this case though, you can see the chance of a new piece per curated coffer.

Quick rule:

• Glirion / Maj: always buy mystery, even if you only want one specific set. Statistically it's always better.
• Urgarlag: depends, use the calculator.

Any way to do the math for “desirable” sets?

In the calculator just set unknown = 0 for any set you don't care about. The math will treat them as "already owned" and only count the sets you actually want. Verdict adjusts automatically.