Willa Shilira is the first "tier-2" to learn Prestidigation to the point of being able to complete a full acid production cycle - following Keltham, Avaricia, Carissa, and Meritxell among the tier-1s.
Seems like a fine occasion to promote Willa Shilira to tier-1! To be clear, that doesn't mean everyone else gets to be tier-1 as soon as they master Prestidigitation to that same level; this reflects an expectation that Shilira will continue to learn at this speed.
Shilira's tier-1 pay is made retroactive to when she was hired; Keltham wasn't quite sure of her value at the time she was hired, but now he's sure, and be it far from him to dock her pay for his uncertainty.