Double Chance multiplies hit damage, it does not add a chance (the skill name is misleading).
It multiplies the damage several times, with level 1 dealing 10% of the initial attacks damage several times (2-9 times, it doesn’t work with numbers 1 and 0 unless you’ve got the attribute Double Chance:Minimum Correction, then it will at least multiply every hit 3 times, even without casting Gematria and Notarikon before).
Gevurah/Geburah also multiplies damage, but instead of multiplying one hit several times, it multiplies several hits one time. The maximum damage boost you can get from this is 2,2 times the damage at level 6 for 0-9 hits (again, if you have the number 0, the skill does nothing, except if you’ve got the attribute Gevura:Minimum Correction, then it will always multiply at least 3 hits).
Note that Gevurah does not work together with innate skill multipliers like e.g. Effigys 3rd hit multiplier.
It will in these cases still use up a charge/hit count of the debuff, but won’t multiply the damage further.
Both skills have the general problem that they trigger on anything that deals damage, so mind your companion, summons, Revenged Sevenfold, autoattacks and/or those of other players (especially bad in crowded situations like dungeons/demon lord battles/guild bosses), so they are not very useful if triggered at the wrong time.
Don’t bother using them if there’s an autoattacker in the party (e.g. those cancerous Quarrel Shooters), it’s a waste of your time [skill delay] and SP (especially with the attributes active,the skills cost a lot of SP, ~500-700, just to multiply the damage of a summon or autoattack is a waste).
All in all Kabbalist C3 is really bad, it deserves a lot better than that to be actually worth a Rank 7 Class that occupies your current 3 final Ranks.
The one thing it excels in is survivability (Ein Sof and the Class Attribute Kabbalah Garments will make you into an HP-sponge), but it doesn’t change the fact that it simply lacks damage options currently (will become a lot better from Rank 10 onwards when people can advance into Inquisitor,Taoist,Zealot and/or the new,yet unrevealed Rank 8 Cleric Class) to be a good round-up of your build.
Also, full support builds are not viable in the game. They are good for certain endgame content [e.g. Belcoper Nest], but they are a waste of ressources [e.g. attribute points] as they can’t be effectively mained [dead partyplay outside of challenge mode&demon lord hunting] or solo farm equipment…