The way it's supposed to work is you just round down essence to get Magic/Resonance.
If your essence drops from 6.0 to 5.8, your Magic/Resonance drops to 5. If your essence drops again from 5.8 to 5.1, your Magic/Resonance still remains at 5.
Initiating/Submerging of course decouples Magic/Resonance from Essence, so it's not exactly that simple. But that's the principle.