C12 should be 22nF.
Here's how that went down... <deep breath>
A few years back another member traced that one and sent me the schematic. On that schematic C12 was marked as 22nF and that's what we went with.
A year or two later Aion released the Delta Amp project where that capacitor is marked 2.2nF. Several people contacted me asking if the Six String project was incorrect, so I contacted the person who originally traced the pedal... He said that he believed 22nF to be correct, but could have been mistaken. Since Aion's trace was more recent (and there was some doubt) the capacitor was changed to 2.2nF.
Cut to a few months ago, I personally traced both the Steel String and the Steel String Supreme. The value was in fact 22nF in both pedals, as originally marked, so the project was (finally) changed back to the correct original value.