When you have a 0 or a negative number in the Base Level entry and use the Misc or something to handle determining your spellcasting caster level it won't calculate your Concentration Check correctly at all.  In fact I can't even revert the sheet to having a concentration check total bonus to 0 once having put at least a 1 in the Base Level entry.  No amount of tweaking or testing has been able to fix this thus far, including the recalc button.  To be fair I noticed this issue a long time ago just never got around to reporting it.  My current workaround for stuff that isn't synced to a class has been to include a Base Level of 1 and put a -1 in the Misc section as well as whatever else I'm using to calculate caster level, or to just manually adjust the Base Level as appropriate.  It doesn't work when synced to a 0 level class however as it will revert to a 0 whenever you recalc.