Unless I misunderstood how Drain Bonded works, you have to recast just the very same spell already casted with a slot, including the level. For prepared this is easier, but in the case of flexible spellcasters, each spell in the Collection is signature, so you can cast it at any level. In this case I think is not easy to track what spell you casted for each spell slot used, as is not like with prepared that you directly indicate in the own prepared spell the level.
The only way I can think about is annotating each spell you cast, but when you have many slots at high level it can be a hard bookkeeping work in a character daily basis.
Some other efficient way for doing this?
Man, I really hate to be this person but... a notepad and pencil. Anyone not taking notes during their game is doing themselves and the whole group a disservice.
Obviously I know and already mentioned (taking annotations), but that can be burdensome in a daily basis. That's why I ask for "other efficient way".
That tracking can get crazy for an Universalist using Bond Conservation. I usually cast the same spell again quickly for a control type spell like slow and otherwise save bond conservation for a damage spell like magic missile.
I write the info down. "Lvl 5 Magic Missile"
Arcane Bond tracking can be kind of a pain, especially if you build it out.
As a DM I'm not too hard on arcane bond. It won't break anything if you're slightly off.
In fact Universalist Flexible with Bond Conservation. I even considered allowing to use Drain Bonded just with the spell already casted removing the exact level requirement (I really don't think it breaks much) but think I have some way to track it.
Thanks for answers.