A spell within Frieren is constructed to do very specific things, flight being the obvious example where it has established constraints, benefits, and so forth, they work within these limits and rules don't stray outside of it, some spells can take less mana yet be exponentially stronger than a weaker spell that takes more mana, or a spell can do absolutely nothing in terms of offense and cost an exorbitant amount of mana, and the opposite is true too. If we legit want to calc his curse, it'd simply apply to his highly specialized curse that he spent centuries optimizing and working on without very explicit proof otherwise that it actually scales to stuff.