There appears to be some sort of bug with how much energy cyclic spells pull from cambrinth. I'm not quite sure, but I think that the number you specify in the INVOKE only applies to one pulse of the spell. Here's what I did.

-Cast Aesandry Darlaeth at 20 mana

-Charge my worn cambrinth armband with 7 mana

-INVOKE ARMBAND 6 CYCLIC, which gives the following message: The cambrinth armband pulses with Life energy. You reach for its center and forge a magical link to it, attempting to ready 6 mana streams for your use. You manage to attune yourself exactly as you intended to. By directing your focus, you ensure that mana is only drawn during the upkeep of spells.

-As expected, I get "Your cambrinth armband thrums softly as it discharges some of its power to maintain your spell."

-Charge the armband 7 again before the next pulse. There should be 8 in there now.

-On the next pulse, I get "Your cambrinth armband thrums softly as it discharges all its power to maintain your spell."

So the first pulse only took the amount I specified, but pulses thereafter took as much as they could to try and get up to 20. The same thing happened with other cyclic spells (Regenerate and Guardian Spirit). I have 484 Arcana ranks and the following techniques: Improvised Rituals, Injured Casting, Deep Attunement, Raw Channeling, Efficient Channeling, Efficient Harnessing and Dedicated Cambrinth Use.

Thanks,
-Life Sustainer Karthor