Ok so just make it appear when the demon really wants something. So i'm going to guess that demon magic mostly works by using the demons power as my character is descended from demons i'm going to guess from what you said i would have enough demonic power for demon spells but not up to the ability that channeling a demon would bring me?
If we go into the specifics with how the mana system works: (This won't make much sense till I post the whole thing.)
As a descendant of a demon, you produce a certain amount of demonic mana, or DEMM hourly. Channeling means a demon gives you a certain amount of DEMM upon request. How the channeling system works is: Channeler makes request, request is detected by granter, granter decides if it wants to grant the energy, if no then nothing happens (If the granter likes you then it will normally be a yes.), if yes then the channeler receives an influx of energy. Corporal beings have mana caps for various types, your character would have a low to medium cap for DEMM and a lower one for other types at the start. Channeled mana doesn't count towards the cap provided it is used within the next two turns, if it is not used within the next two turns then it counts towards the cap and has a chance of overcharge, overcharge is the same kind of thing as what happens when a Lunarik discharges, you don't want to be near it and it isn't normally good. If you survive then you will likely be manaburnt or manalocked, often both. Those can result in more overcharges, which again are really really not good.