To me, it seems like Levemir has a flatter curve than Lantus, as long as the cat isn't high dose or having other complications. That would explain why you aren't getting very low.
You may need to wait a few days for any "New Dose Wonkiness" to clear. After 5 days or so, a nadir will let you know if there may be room for another increase.
How's he doing otherwise?
The 5 Ps + intake:
Peeing & drinking
Pooping & eating
Playing
Purring
Preening/grooming