FLT CREW OF DH8C FAIL TO SET 29.92 PASSING FL180 AND LEVEL AT FL205. ATC ALERTS BEFORE CONFLICT.

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WE LEVELED OFF AT 16000 FT ON AN ALTIMETER SETTING OF 29.43. DUE TO DARKNESS; WE COULDN'T TELL AT FIRST THAT WE WERE LEVELING RIGHT AT THE TOPS OF A LAYER. WE BEGAN GETTING CONTINUOUS LIGHT CHOP; SO WE REQUESTED FL200. THE ALT WAS GRANTED; AND MY FO BEGAN THE CLB. USING THE AUTOPLT HE CLBED AROUND 2000 FPM; WHICH IS GREATER THAN USUAL; ESPECIALLY AT THAT ALT. THIS HIGH RATE OF CLB CAUSED THE FORWARD OUTFLOW VALVE TO OPEN. THIS MAKES A UNIQUE SOUND; SINCE IT IS ON THE FORWARD BULKHEAD; CAUSING US TO LOOK UP AT THE PRESSURIZATION PANEL OVERHEAD. WE THEN BEGAN DISCUSSING WHY IT OPENED; AND DETERMINED IT WAS APPROPRIATE SINCE THE CABIN WAS AT MAX DIFFERENTIAL. WHILE TALKING; WE MISSED OUR STANDARD '1000 FT TO GO' CALL; AND OUR '29.92' CALL. WE LEVELED AT FL200 AND SET CRUISE PWR. A FEW MINS LATER; ATC CALLED AND SAID 'SAY ALT.' I LOOKED AT THE ALTIMETER; THEN AT THE ALTIMETER SETTING AND SAW IT WAS STILL AT 29.43. I QUICKLY SET IT TO 29.92 AND SEEING WE WERE 500 FT HIGH; STATED 'DSNDING BACK DOWN TO FL200.' ATC THEN STATED; 'ROGER; MAINTAIN FL200 FOR TFC.' EVENT OVER. NO TFC CONFLICTS.