DISTRACTED BY OTHER EVENTS; PC12 FLT CREW MISSET ALTITUDE CONTROLS AND DESCEND THROUGH CLRED ALT.

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DURING OUR DSCNT FROM CRUISE ALT INTO THE TERMINAL AREA OF CVG WE WERE INSTRUCTED TO LEAVE FL260 FOR FL240. THE ALT CTL FOR THE AUTOPLT IN THIS ACFT HAS 2 KNOBS LIKE A COMS RADIO. I SET THE NEW ALT INTO THE AUTOPLT WITH THE BIG OUTER KNOB (THOUSANDS OF FT); THEN PULLED THE INSET SMALLER KNOB OUT TO SET OUR RATE OF DSCNT (HUNDREDS OF FT). DUE TO A BAD GLARE ON THE DISPLAY FROM THE SUN I DID NOT NOTICE THAT I HADN'T PULLED THE SMALL KNOB OUT COMPLETELY BEFORE SETTING THE VERT SPD MODE AND INADVERTENTLY CHANGED THE ALT ITSELF TO 23800 FT. DURING THE DSCNT I WAS DRAWN INTO A CONVERSATION WITH MY COPLT EXPLAINING THE PRESSURIZATION SYS ON OUR ACFT. AS WE DSNDED THROUGH FL240 I SAW THE ERROR AND THOUGHT WE WERE SUPPOSED TO GO DOWN TO FL230; SINCE THE BUG ON THE ALTIMETER WAS ON THE 3000 MARK; OUR INITIAL ALT AT OUR DEP ARPT. WHEN ATC ASKED US TO VERIFY OUR ALT I IMMEDIATELY NOTICED THE PROB AND PREPARED TO CLB BACK UP TO FL240. THE CTLR ADVISED US NOT TO WORRY ABOUT IT AND WE CONTINUED OUR DSCNT INTO THE TERMINAL AREA. CONTRIBUTING FACTORS WERE MY INCORRECT INPUT INTO THE AUTOPLT; LIGHTING CONDITIONS IN THE COCKPIT THAT MADE VIEWING THE DISPLAY DIFFICULT; AN INEXPERIENCED COPLT WHO EITHER DIDN'T NOTICE THE ERROR OR WASN'T COMFORTABLE NOTIFYING ME OF IT; AND MY DISTR DURING THE DSCNT. CRM WAS A MAJOR ISSUE SINCE I HAD NEVER FLOWN WITH MY COPLT BEFORE AND SHE HAD VERY LITTLE EXPERIENCE IN THE ACFT. THIS INCREASED MY WORKLOAD BECAUSE I HAD TO EXPLAIN HOW AND WHY EVERYTHING WAS BEING DONE. DUE TO THIS INCIDENT I HAVE LEARNED TO BE MUCH MORE CAREFUL AND PRECISE WHEN INPUTTING NEW ALTS OR HDGS; TO VERIFY WITH ATC ANY TIME SOMETHING LOOKS OUT OF PLACE (SUCH AS AN INCONGRUOUS NUMBER LIKE 23800); AND TO MAKE SURE MY COPLT; REGARDLESS OF EXPERIENCE; IS COMFORTABLE QUESTIONING MY ACTIONS OR JUDGMENT. AS PIC I NEED TO WORRY MORE ABOUT FLYING THE ACFT AND LESS ABOUT BEING A FLT INSTRUCTOR TO A NEW COPLT.