You're right about the math, but in that other thread, we don't know what the vials are *supposed* to be. Maybe theirs were supposed to be 8mg and were overfilled to 10mg - same difference in percentage.
The good thing about that thread is they tested four vials, which tells you if the whole batch is consistent. One vial only tells you about *that* vial.
If you tested four of your own vials and they ranged from 24-30mg, you'd know you might sometimes be underdosing. If they were consistently 29-31mg, you'd feel better about using your 30mg result. But you can't know with just one test. Testing isn't cheap, I get it.