Small Question of Logic about Tear of Goddess Item

It just randomly came to my mind but Apparently (And i tested it in practice tool just to be sure) The passive effect that you get 15% of your used mana refunded is post-active calculated (IDK if this is a term but whatever)

Wouldn't it make more sense for the mana to automatically calculate how much is refunded and spend the appropriate ammount instead?

Take Ryze for example. his Q costs 40 Mana. With tear, it technically costs 34 Mana since 6 mana is refunded 6 being 15% of 40. BUT as of right now, you still spend 40 Mana and just get those 6 refunded after using the Skill.

Wouldn't it be slightly easier for mana management if instead the refund calculation would occure BEFORE the skillcast? so you directly invest 34 mana instead of 40 and have those 6 refunded mana automatically calculated into your manapool.

IDK, maybe it sounds like a stupid Thing, but in my personal opinion it would make tear a Little easier to use or at least simplify mana management by a Little.